A.C.
You told him you weren't going for child support but he told you he was going to support them. If he is not supporting your children, then yeah, make it legal. It's not about you. It's about the quality of life for the children.
***edited after reading some comments: Hmm. I just take for granted that everyone is in the same situation that my family was in. My dad had to pay my mom $1,000/month for child support for EACH child, and that was in the early 90s. Yes, get the forms and see what he would owe as of now. Perhaps free childcare is something to consider. But if you do this legally, then MzKitty is correct and the childcare, insurance, ballet practice, whatever you guys agree to is considered---that's what my divorced bff's child support includes, at least. Child support isn't for living the high life, but you don't want your child to do without opportunities, medical care, clothes, school supplies, a sports league, simply because you can't do it all on your own salary and he isnt stepping up. Doing it legally protects you in the long run. He may have good intentions, but my experience screams that men's good intentions don't run the whole course of childhood (heck, your own experience says he paid what? one month??)