I understand that Aero and this is not my personal opinion, just saying how it might look from dss point of view.
How old was he when his dad left?
Whatever age he was, his dad left and had little to do with him, for whatever reasons, from the child's point of view he is becoming estranged from his father.
When is is 16 his father married a woman who already had a four year old son. At this stage, the little boy is really nothing to do with him. Then OP says that the 16 year old began to resent the relationship his dad had with this woman's child. Something that he didn't have. It might have seemed that the boy was 'stealing' his dad.
All this tiime, he was still a child himself, coping with difficult circumstances so, yes, his view has been skewed.
Now he is getting married and they can have x amount of guests. They have to make sacrifices somewhere, maybe some of their friends can't come or some of their other more distant relatives, who knows.
But it boils down to the fact that they need to restrict numbers. He can't have all four of the 'step family' but he really wants his dad to be there on his special day.
Weddings are a nightmare at the best of times, you have to draw the line somewhere, so his line was close (blood) family members only, or whatever it was that brought him to this conclusion. And don't forget it wouldn't be his choice alone, there is the bride to please too 
He may have found himself between a rock and a hard place. He may have been pushed into this decision by other influences in his life. We don't know, we have no idea of his motives.