How do Lutherans have their sin forgiven? If a Lutherans sins, how is his sin forgiven?
Must he confess?
Is there a distinction between venial and mortal sin?
What if he sins again the same sin?
Lutherans have their sins forgiven after confession to a pastor who can absolve the penitent.  Here is an extract from an article that discusses the subject at some length:


  The Lutheran Church practices "Confession and Absolution" [referred to as the Office of the Keys] with the emphasis on the absolution, which is God's word of forgiveness. Indeed, Lutherans highly regard Holy Absolution...  In confession, the penitent makes an act of contrition, as the pastor, acting in persona Christi, announces the formula of absolution. Prior to the confession, the penitent is to review the Ten Commandments to examine his or her conscience. In the Lutheran Church, like the Roman Catholic Church, the pastor is bound by the Seal of the Confessional. Luther's Small Catechism says "the pastor is pledged not to tell anyone else of sins to him in private confession, for those sins have been removed." If the Seal is broken, it will result in excommunication.
  
  In line with Luther's initial statement in his Large Catechism, some Lutherans speak of only two sacraments, Baptism and the Eucharist, although later in the same work he calls Confession and Absolution "the third sacrament." The definition of sacrament in the Apology of the Augsburg Confession lists Absolution as one of them. Luther went to confession all his life.
  
  The Lutheran reformers held that a complete enumeration of sins is impossible and that one's confidence of forgiveness is not to be based on the sincerity of one's contrition nor on one's doing works of satisfaction imposed by the confessor (penance)...  The Lutheran reformers abolished the "satisfaction of deeds," holding that confession and absolution consist of only two parts: the confession of the penitent and the absolution spoken by the confessor.  Faith or trust in Jesus' complete active and passive satisfaction is what receives the forgiveness and salvation won by him and imparted to the penitent by the word of absolution.  Source: https://en.wikipedia.org/wiki/Confession_(Lutheran_Church)


Another article, “What about… Confession and Absolution?” by Dr. A. L. Barry, 10th President (1992-2001),the Lutheran Church—Missouri Synod, has this to say:  


  Confession has two parts. First, that we confess our sins, and second, that we receive absolution, that is, forgiveness, from the pastor as from God Himself not doubting, but firmly believing that by it our sins are forgiven before God in heaven...  Absolution is the ongoing work of Holy Baptism, in which our old, sinful nature in Adam is drowned and the new man in Christ arises. Through Holy Absolution we receive “the gift of God,” which is forgiveness of sins and “eternal life in Christ Jesus our Lord” (Rom. 6:23)...  Confessing our sins in the Divine Service, we hear the Lord’s servant, our pastor, absolve our sins in the name of Christ. Privately, we go to the pastor for confession and absolution precisely for those sins we are most aware of and those sins that are particularly troubling to us. These we confess to our pastor and hear the words of Christ, “I forgive you.”
  
  At times, our pastors, with considerable sorrow, may have to tell a person not to attend the Lord’s Supper until he or she has repented of sin. If the situation continues without repentance, the pastor may have to declare to the person, on behalf of the congregation that has made this decision, that he or she is excluded from the church until there is repentance.  Excommunication is a last resort to help a person recognize the extremely dangerous situation he has placed himself in because he will not repent of his sin. It is a final attempt to win someone back from Satan’s influence.   Source: https://www.faithlutherancorning.org/confession-absolution


The Lutheran Church believes that repeated sin hardens the heart and may lead to sinning against the Holy Spirit.  Also, as noted in the previous extract above, unrepentant and repeated sin could result in being excommunicated:


  SIN: Transgression of God's law (Ro 4:15; 1 Jn 3:4). Sin may be divided into original sin (see Sin, Original) and actual sin. Actual sin (every act, thought, emotion (e.g., lust*) conflicting with God's law) may be involuntary or may be done ignorantly (Acts 17:30) and includes sins of commission (cf., e.g., Mt 15:19; Ja 1:15) and sins of omission (Ja 4:17). Sin arouses God's righteous wrath and deserves His punishment. Willful sin sears conscience*; repeated, it hardens the heart; may lead to, but is not identical with, the unpardonable sin against the Holy Spirit.’  (http://cyclopedia.lcms.org/display.asp?t1=s&amp;word=SIN


With regard to venial and mortal sin, there is no mention of venial sin, only that all sins manifest total corruption and are deadly:


  VENIAL AND MORTAL SINS: The Lutheran Confessions speak of sin* that is mortal, or deadly, i. e., irreconcilable with faith (Ap IV 48, 64, 109, 115). When believers fall into open sin, faith has departed (SA-III III 43–44). One who obeys his lusts does not retain faith (Ap IV 144). Original sin (see Sin, Original) is mortal; it brings eternal death on those who are not born again (AC II 2 Lat.). One who is dead in sin is insensitive to sin (LC, V: The Sacrament of the Altar, 77–78). Sins remain in believers (SA-III III 40; FC SD II 34). Many regard the following as 7 deadly sins, fatal to spiritual progress: pride,* covetousness,* lust,* anger, gluttony, envy,* sloth. But man cannot weigh, distinguish, or differentiate sins; all sins manifest total corruption (SA-III III 36–38), merit God's wrath (Mt 5:18–19; Gl 3:10; Ja 2:10), and are deadly (Eze 18:4; Ro 6:23); every sin loses its deadly effect when Christ, apprehended by faith, intervenes (Ro 8:l; 1 Jn 1:7, 9; 2:1–2). Source: http://cyclopedia.lcms.org/display.asp?t1=s&amp;word=SINS.VENIALANDMORTAL


Another source of information said this:


  The Forgiveness of Sins – Introduction: The Bible reveals that we are accepted by God through the forgiveness of sins in Christ.
  Conclusion: God has declared the world righteous in Christ (objective justification).  He justifies each sinner personally and grants the forgiveness of sins when the Holy Spirit gives faith in Christ (subjective justification).  Although the believer will never be entirely free from sin in this life, the believer wants to turn daily from sin to good works.  The Holy Spirit renews the believer with a daily sanctification so that in the new life of faith, the Christian can offer good works as a thank offering to His Savior God.  Source: https://havasulutherans.org/catechism-explanation/forgiveness-sins/


Finally, I found a 2007 article from the official newspaper of the Lutheran Church, Missouri Synod, which answers the question, “What Gives the Pastor the Power to Absolve?  https://blogs.lcms.org/2007/what-gives-the-pastor-the-power-to-absolve-5-2007/