Why do you want them to? It's okay for your boyfriend to have "his world", you shouldn't need to be part of everything in his life. Give him that space.

Play it the other way:

Do you want your boyfriend to be close/friends with your girl friends? (i.e. not just "friendly"). Do you want him to be texting one of your girl friends about their shared hobby with you out of the loop? If you and your boyfriend are fighting temporarily, do you want them to continue talking to your boyfriend normally? How about if he breaks up with you (or you break up with him), do you want any of your girl friends to continue being close and friends with your boyfriend?

If you answer "No" to any of the above questions, then you have your answer to your original question.

The relationship his friends have with you are based on him and through him. Just like his relationship with your girl friends go through you.

It's okay, and good, to keep these separate.