What happens when one returns to faith? In the Catholic tradition, what happens to one who grew up a faithful Catholic, apostatized, and then later returned to faith? What would the process of returning to the church look like? (i.e. Would they need to be re-baptized? Would they have to go through RCIA?)
No, they would not be re-baptized (unless there was doubt about the validity of the first baptism, in which case he would need to be conditionally baptized).

In the case of a public apostate, it would be prudent for a bishop to issue a public letter making the ex-apostate's reversion known to all, so as to avoid the possibility of scandalizing those who might think he is still excommunicated. Oftentimes, the penance for a public sinner is public, so his reversion could be manifested that way as well.