Like Cleveland Mom suggested, I'd seek counseling for him.
This may not be the case with your son, but my friend's son feels like he isn't really part of either family (his mom is remarried with a child and his dad is remarried with two children). He seems to feel like he's floating between two nuclear families and he doesn't really "belong" to either.
And I'd be very careful not to suggest he go live with his dad (especially out of frustration or anger). And be careful he does't overhear you talking about your struggles with him.
Well, I was a single mother and I never felt I was stuck in single mother hell....
The key thing here is to come up with a game plan with your ex. The two of you need to make a list of rules and consequences that will apply at both houses. It is stressful on the kids, moving between two families. Often they feel that they don't belong anywhere. If the parents are a unified this should alleviate a lot of the problems.
Consistency, consistency, consistency. And if at all possible, have a positive relationship with all the adults in his life.
Yours may be a situation where family therapy could be quite beneficial. Chances are if you let things run its course with out really trying to make changes or address issues with your son, you'll look back at this time with regret. A family therapist can help you sort out the issues and address them in the most effective way. In short, a therapist can help you get your family on track. It takes a good therapist and good efforts on the part of you and your family. You can do it. Talk to your pediatrician or family doctor about a referral. Best wishes...
You're stuck in single mother hell.
Your husband feels too guilty because of the mistakes you've both made to discipline him, and you are kind of unable to discipline him because boys need dads to do that effectively, by example.
I hope you get better advice than I can give you, because I've seen this happen over and over, and I can't think of a way to make it better.