Reddit mentions of Stella Brings the Family

I guess he's a bit old for picture books, but there are a couple fantastic ones that address this issue - Stella Brings the Family features this exact situation about a child not wanting her two dads to attend a social event at school.

Maybe talk about the diverse family structures of other kids in his school (single parents, adoptive parents, grandparents or other relatives standing in for parents, maybe foster parents, and very likely some other same-sex parents!). You can point out that he's showing other kids it's okay to have a non-stereotypical family, and that if everyone made their same-sex parents stay home from events, kids at school might never know that it's possible to have a fantastic loving family with two moms!

At the end of the day though it's up to him, and you can emphasize that you'll respect his decision, you'd just like him to think about it and make sure that's really what he wants. You can't promise there won't be any teasing, because kids can be idiots, but if he has good friends who understand, maybe that's a risk he could be willing to take.