If you don't trust him you got a problem..
Also it's completely normal to dream about others even tho you're in a relationship, he's just unlucky he's sleep talking, or you are, or both of you really..
I'll follow up with this; if you really like your boyfriend, follow this 5 simple steps and he'll love you back nomatter how hot the girl he fucked last year is.
1 - cook for him
2 - give him a bj
3 - if he wants to chill with he's boys, let him chill with he's boys
4 - give him a bj, again
5 - Don't nag
Good luck!