Hi T., I'm on the other side of this situation. My husband has a son from a previous marriage who lives with his mom three hours from us. My husband loves his son dearly but rarely calls and never visits. I have to remind him to call or ask when we're going out to see him, or he is coming to see us. (When he does come to see us I make all the plans or we would do nothing while he was here. I want the short time he spends with us to be special.) I try not to step on my husbands toes too much but do bring it to his attention. My husband does not intentionally "forget" about his son, it's just the kind of person he is. He is a bit emotionally detatched. I even have to remind him to say Hi to our girls when he gets home sometimes! As far as child support, I make out the check or it would never get there. Again my husband does not do this out of dislike for his son, he just doesn't think about it.
I think my husband only has the ability to see what is in front of him (me and the girls.) It's not fair or even okay, but that's the way it is and we cope with it. In order to cope with it we all have to play a role. This is the only way to make sure his son has a relationship with his dad and sisters. We do what we have to do for our kids.
Maybe you could send his new wife an email and talk to her about the situation. Maybe she would be willing to step up and help come up with a schedule where father and son could see eachother. As a step mom I imagine she would be happy for her kids to be able to know their brother better.
I just don't want you to protray your son's father badly in front of him because that too is not fair. He may have a great daddy who needs a little push.
Good luck with your situation.