I don't see how he is being a simp for Grace. Getting together with Grace you know that the kid is a package deal and something you have to accept not being a simp at all. Anyone that actually cares for a single mother beyond sex but actually loves or cares for her deeply wouldn't have issue with it ( assuming the kid isn't a deal breaker for them), it's just being mature and a adult to be with the one you love even if it isn't ideal.
Sure it is or isn't a deal breaker for some. He is literally just reconnecting with a old friend , which might include looking after the kid once in awhile as a favor but that's called being a good friend. She isn't trying to take advantage of him or anything and the MC sure as hell isn't simping over her, the MC is just acting like an adult. I'm sure if all she did was use to him to take care of the kid then he would stop being a friend at one point. To me this reads / sounds like you just have a personal problem with her character having a kid that isn't the MC's for some reason when you can chose not to play her path.