My ex isn't even in the picture. DS1 hasn't seen him since last Christmas Eve, and that was only because my ex showed up at his sister's (ds1's aunt) place for a free meal. DS1 didn't even know he'd be there. Before that, it was New Year's Day. While I'm just as glad to have my ex out of our lives, it's been really hard for ds1 to understand that the dad he had for his first seven years of life chose to simply disappear.
I don't know how long you've been split or anything, but if it helps - ds1 has mostly adjusted to hid dad's crap. He's got a loving, wonderful, engaged stepfather, who takes an interest in his life. As time's gone on, he's begun to understand that dh chose to get involved with us, not just me...that he knew ds1 when we got married, and wanted him as a stepson. It's taken a long time (they hit it off right from the start, though), but ds1 doesn't mourn his dad like he used to. It's never going to go away, but it's something he's learned to live with and more-or-less to accept.
Your poor ds doesn't deserve this crap. I hope he understands, someday, if not now, that this is his father's issue, not his. DS1 doesn't look for interaction with his father, anymore, because he knows he's not going to get it.