well, i'm going to be in the minority here, and suggest that you leave the door open for him. maybe tell him to write letters, not emails, to your son, and let them get to know each other that way. you should also make it clear to him that he must respect your rules and parenting choices.
he's been a giant ass, there's no doubt about that, but maybe it took your recent marriage to turn a light on in his head and say, "oh crap, i have a son out there, and another man will be raising him, making me obsolete." perhaps he thought that as long as you were a single parent, his place as father was secure.
your son is old enough to see when someone is a loser.
it always hurts me when i hear of parents not seeing their kids, either by choice or not. my sister has been a single parent for most of her life now. the biodad denied that her children were his, and he has approximately 20, yes 20, other children with various mothers. my nephew used to cry every father's day. now he is 15 and his attitude is, if he doesn't want me, i don't want him.
i think if your ex is as big a loser as you describe, than it will not take long for your son to figure this out. if you keep them apart, or tell your son his biodad is worthless, you'll be the bad cop. give him enough rope to hang himself, right? think about it, your son is old enough to ask him where the hell he's been for 8 years, why didn't he call, or write, etc. your son can hold him accountable, and if he's like most kids, he won't pull any punches.
and you never know, he might just surprise everyone, including himself, and be able to at least maintain a relationship with your son. or he may make a giant ass of himself and your son may tell him to get lost.