Well if you want to talk to him first, then I'd do just that. Do you all text?
I would just say exactly what you said here, "I was looking for some tax stuff and I realized that the support you're supposed to pay me is X amount and not Y amount and that started last year." and see what he says.
If he gets pissed, then it's time to involve the lawyer. I hope it works out for you!
Ugh, this is so hard, I'm sorry I don't really have any experience, so no advice, but I know my mom had to make the choice to miss out on $$ and avoid court to keep my dad from trying to duck with the custody arrangement and such. I can't imagine how frustrating it all is.
I guess I'd just try to be as non-attacking as possible. Come at it like, he must have forgotten, you did too, y'alls bad, but the money is for your daughter so how does he want to handle it? So the assumption is that he WILL handle it, but you're giving him some control. Like with a toddler, lol.