Mike must be the man he betrayed, and keeping updated on his life on a dead, old desire for revenge is pretty funny when you accept that he clearly will probably never do anything about it…?
Man betrayed someone and they are doing well. He will probably want to betray the son and help him into grad school just like his father...through recovery of the betrayal.
So, Will betrayed his friend. Eventually that friend got married, had a kid, and went to grad school. Will sees this on Instagram and thinks "Wow, my betrayal must have had a positive effect on them. If I betray their child it might have the same effect."