According to Jehovah Witnesses, in what way is Jesus human? From what I understand, Jehovah Witnesses believe in a form of Arianism that claims Jesus is “the son of God”, but he is inferior to “God the Father”, as he is usually understood as “God’s first creative act”. Then this spiritual entity was sent to earth, where he “became flesh” in the “incarnation”. 

I’m not a Trinitarian myself, but it does include words like “human nature” and “hypostatic union” to describe how Jesus can be a “real human”, while “consubstantially” existing as something much different than a human.

According to Jehovah Witnesses, in what way is Jesus human? Was he a spiritual entity cloaked in human biology, or would they agree with the Trinitarian idea of a hypostatic union?
The Jehovah’s Witness religion teaches that there was a time when God was utterly alone and decided to create, as his first creation, a being known in the Bible both as the Word of God, and Michael the Archangel.  Then the by means ifWord / Michael, Jehovah created all the other angels plus this universe and planet earth and all life on it. He is said to have been subordinate to God in this vast project, only God never having been created. So, the first point to grasp in order to see what they believe about the Word becoming flesh (John 1:14) is that this created Son of God later became Son of Man, born to the virgin Mary as a perfect human being.
This is what their ‘Bible Teach’ book says about Jesus and Michael the Archangel (pages 218-219 in the Appendix) followed by a quote from their ‘Insight on the Scriptures’ Vol. 2 p394:

&quot;The Bible indicates that Michael is another name for Jesus Christ,
before and after his life on earth… It is logical to conclude that
Michael is none other than Jesus Christ in his heavenly role.&quot; “In his
prehumen existence Jesus was called ‘the Word.’ (Joh 1:1) He also had
the personal name Michael. By retaining the name Jesus after his
resurrection (Ac 9:5), ‘the Word’ shows that he is identical with the
Son of God on earth. His resuming his heavenly name Michael and his
title (or name) ‘The Word of God’ (Rev 19:13) ties him in with his
prehuman existence.”

They claim that it was the spirit of Michael that entered into Jesus when he was born and that the body of Jesus was never resurrected. Instead, what came out of the tomb was a spirit creature (who transformed himself into human form to make the disciples think he was human although he was actually, in their belief system, a freshly created spirit). And it is Michael the Archangel who is coming back at Armageddon, they say.
This leaves us with the question as to who they say Jesus was, humanly speaking. Given all that pre-human angelic status, was he  half-human, fully human or more than human? The same ‘Insight’ book says, p56:

“Since actual conception took place, it appears that Jehovah God
caused an ovum, or egg cell, in Mary’s womb to become fertile,
accomplishing this by the transferal of the life of his firstborn Son
from the spirit realm to earth. (Ga 4:4) Only in this way could the
child eventually born have retained identity as the same person who
had resided in heaven as the Word, and only in this way could he have
been an actual son of Mary… It is likely, therefore, that the child
born resembled its Jewish mother in certain physical characteristics…
it would appear that the perfect male life-force (causing the
conception) canceled out any imperfection existent in Mary’s ovum,
thereby producing a genetic pattern (and embryonic development) that
was perfect from its start. Whatever the case, God’s holy spirit at
the time guaranteed the success of God’s purpose… he was perfect in
physical form… Hence he had not simply materialized a human body as
angels had previously done; he was not an incarnation but was actually
a ‘son of mankind’ through his human mother. (p 1002)”

This shows that they believe Jesus to have been fully human, and a perfect, physical man, not just appearing to be human (except at his resurrection when he had been re-created as a spirit who had the ability to appear to be physically human.) But was he more than human? Apparently not given what they say about denying he was an incarnation, and also claiming Jesus was an exact equivalent of the man Adam, prior to him sinning.  Again, quoting from that ‘Insight’ book (p 736):
“The related word antilytron appears in 1 Timothy 2:6. Parkhurst’s Greek and English Lexicon to the New Testament says it means:,'a ransom, price of redemption, or rather a corresponding ransom. …So Aristotle uses the verb [antilytro’o] for redeeming life by life.” (London, 1845, p. 47) Thus Christ “gave himself a corresponding ransom for all.”
In their book You Can Live Forever in Paradise on Earth, p 62, they claim that Jesus is the only man who ever lived that was equal to Adam as a perfect human son of God, and on the next page they have an illustration of a pair of balanced scales, Adam on one dish, and Jesus on the other. Given that Adam was never in heaven nor was an angel, the question remains as to how Jesus could correspond exactly to the man Adam (before he sinned) if Jesus somehow also (simultaneously) had the personality of the angel Michael right from conception. But that is up to the Jehovah’s Witnesses to deal with. I have simply quoted from their literature to offer an answer to, ‘in what ways was Jesus human’. It is very clear that they would never agree with the Trinitarian view of a hypostatic union.