Oh, Stuffinb, I know what you are going through.
I have a nephew that is 16 years old. He just got his first truck, and he has spent every penny he earned last summer fixing it up. He ran into his “father” (if you can call him that) at Wal-Mart the other day. (‘Father’ and Sister have been divorced for about 5 years now.) ‘Father’ says, ‘oh, now you have your own truck, that’s really great, you can come and see me now.’
As if my sister didn’t drop him off whenever Nephew wanted to go there.
As if ‘Father’ ever bothered to come and pick Nephew up, take him to a hockey game, or do anything somewhat fatherly with him.
It’s truly sad when Nephew gets all excited about his dad, because the rest of us know that ‘Father’ is never going to follow through with what he promises. But never once has my sister bad-mouthed him, in front of Nephew, at least. She just tells Nephew that he needs to make arrangements to see his dad, and that she will give him gas money, whatever he needs to get there. She encourages Nephew to call him, try to make plans. Unfortunately, ‘Father’ is just a schmuck, and he will never realize what he’s going to lose in my nephew.
Encourage your kid to write a letter to his mom, so that he can get out what he is feeling. Tell him that you think it will help him a lot, and if he gets to the end of the letter and doesn’t want to send it, it’s ok. As long as there is some kind of release, it will help. And you never know, depending on how old your kid is, he might give you the letter and say “can you read this, to make sure it sounds ok?” which is basically his way of saying ‘ok, I want you to know, too, but don’t criticize or tell me to change the meaning.’ (Which basically means, read it, and tell him it’s good, but hey, you forgot a comma here.)
Good luck. Hopefully your son will realize that sometimes moms just aren’t meant to be moms, and it’s better to have a kick-ass dad like you. 
Oh, and the only thing that I would say to his mom would be “You know, you’re all talk. You don’t bother yourself with the kids, yet you get upset when they disregard you the same way. I wondered where they got that from.”