B.B.
Sounds like your son's not happy having to compete with his step-siblings for his dad's attention.
Kids thrive on structure, and it could be that your ex's household doesn't have that. FURTHER, and more importantly, when your son acts up, your ex isn't handling it appropriately. Your son wants to back home to you, and that's exactly what he's getting. He's acting up just enough to get what he wants, and he keeps "raising the bar" until he gets it.
Your ex and you need to come to an arrangement - if he doesn't want visitation anymore, then set it up in the divorce decree that way. If visitation is going to be maintained, then he needs to deal with the son's behavior appropriately. 8 might be a bit old for "time out" but he could have a "reflection spot" to think about what he's been doing. Maybe if he kept a journal, that might help too.
Wish I could offer more....good luck!