You can always do Yoga with out going to a class. You can do Yoga by yourself, at home, in the mornings when your son is sleeping.
You can also accept that your ex is not going to give you any money for child support and adjust your finances accordingly.
There are many things you can do with your child for free, doesn't cost anything, and it will mean much more to him than anything you would ever pay for. All he needs is you, your love, and your time. That costs nothing.
I hope I don't sound too harsh, but I wanted to point out that you DO have control over a great many things. You just need to look around and take a step back to see it.
I'm sending you my love energy too.