Hi. I used to get the same thing with my DS, he was shy with people he didn't know and "mama's boy" was something I was hearing a lot. It used to bother me, but I decided at some point that I wasn't going to let it any more.
My DS is now 17 months and he is MUCH better with new people. It still takes some warming up time, but that is to be expected with anyone. Oh, I always try not to force DS into anything/anyone he isn't comfortable with.
So, with exposure and time, I am sure your LO will start warming up too! HTH