It's obviously not like a typical father/son relationship. Micheal is at the very least a drop out and Sonny might have threatened him. If this is the case, I could imagine it being difficult to visit a family member who threatens you. Michael has kids of his own, and he's going to look out for them before all else. That being said, Michael just seems like a total opportunistic douchebag so if it's a case of Michael just ghosting Sonny for his own selfish interests, I wouldn't be surprised.

$500/mo. social security. Seems we're taking care of our elders today about as good as we were in the 1930s, which is one of the reasons organized crime groups were formed in the first place.