is my puppy just submissive?

My puppy is a 5 months old jrt, and he's never been afraid of other dogs, even bigger. But when he plays fight with them he has this behaviour that I can't quite understand the meaning of (except with that one border collie). When they play fight he will go belly up with his tail tucked, which I first thought was a sign of him being scared or intimidated, sometimes he'll even come back to me and put his front legs on my leg. But then he always goes back to play fight with the other dogs instantly ? like, does he just submit fast, is he actually scared, does he need to find his confidence ? It's just the tucked tail that I am unsure of.

Also there's this 7 months old Australian shepherd that it pisses off so much haha. when he does that she just barks, and coupled with my puppy's behaviour we're never really sure if we should or when to separate them.