Sorry so late getting back to this one - we were out of town. Some 'men' out there do not like the kids from previous relationships. They see them as a threat because it's a link back to another man, a reminder that their 'woman' was with someone else. This is why they say the non-father boyfriend is one of the biggest hazards in a young child's life. You see so many stories in the news where the boyfriend has killed or harmed the girlfriend's child - probably because he has no vested attachment to that child and subconciously sees at as a rival of the mother's affection. These are men who are unable to control those primal urges. Look in nature and when a new dominant male takes over a wolf pack, pride of lions, etc the first thing he does is kill the offspring off his predecessor. as humans we should be able to look past that and there are many men who make wonderful stepfathers (my BIL is one who is actually a better father than the dirtbag sperm donor who once he found out SIL was remarrying asked when her new husband would be adopting the boy - he was only concerned about not having to be on the hook for any more child support).
It does sound like it is an issue on both sides. I'm sure the stepfather wishes the young man would hurry up and get out of his house so that he no longer has that reminder of the past/competition for his affection.
Seriously though, my cousin almost never spoke to her stepfather. We would visit up there for about 2 weeks at a time, and other than passing on a message about dinner being ready or some other instance where her mother told her to tell him something (or him telling us he wanted the TV), there was just a handful of words spoken between them. We generally just ended up avoiding him and spending most of the time together at our grandparents' house where we wouldn't have to see him.
Unless they both decide to be adult and apologize (which male pride will keep them from doing unless there is some sort of intervention where they are forced to sit down at the table and communicate the source of their disfunction), I wouldn't hold my breath.
The son probably has some resentment about his mother 'replacing' his father. If you've not watched the movie 'Parenthood' I think it does a good job showing the range of personality types you get in people. The young teen boy of the divorced mother does a great job showing how easy it is to retreat from family.
There's not likely anything you can do. If your aunt isn't willing to push both parties to change, they won't. and even if she does, it's no guarantee that both sides would want to come to a more normal relationship. i can understand how it seems odd to you, but to them it has become normal.