hmm....well let him in and see how they react to eachother. The mama cat will probably tell him to go stuff it and smack him around though he's young enough that he'll probably want to play with the kittens.
Domestic cats that have been raised in the house sometimes can be more friendly with kittens. I had a cat to had kittens in my room and my old male cat LOVED them, pissed the mama off cuz he would steal them and put them in his bed then snuggle with them, then she would come over and beat him up and take them back. Then my other cat (female) would just sit and glare, then get up on the top shelf where the kittens couldnt get at her.
All cats react differently, if your boy cat acts jealous of them at all then its best to keep them apart, though if he is nice and just playful then it could be fine. The mama cat might drive him off and smack him around for getting too close, if shes too mean with him you might wanna keep them apart a little longer, really depends on how close they are.