K.N.
The high road, the high road, the high road. Stay on it till your nose bleeds. He is fighting a battle in himself that he probably won't have the strength to win until he is much older. Shy of court action, there isn't much you can do. And if that is an option you are considering, my advice is to document, document, document. EVERYTHING. Every conversation, every unreturned phone call, every missed appointment, everything.
Find a stepmom's group, there are many (you will have to find the one that works for you). This affects you, but it's really for your DH and the ex to resolve.
Then start practicing natural consequences. You don't say how old he is. If he spends all his allowance, well, sorry kid, no more money till the next deposit. HE learns how to mind HIS money. The ex probably assumes you will replace the money he spends on her, so it may not be presented that way to him. Have your DH explain that it's a set amount of money. No replacements, whatever the son chooses to spend it on, that's it. Parenting With Love & Logic is a great resource for natural consequences. There are several age appropriate versions.
And then take care of yourself. It's not easy being a step-parent. There are so many sinkholes that you could unknowingly step into. Learn to detach. It's your house, unfortunately, you don't always get to call the shots when you're a step-parent. Deep breaths, the high road and taking care of yourself.