I have read that 3 months is when they start to realise anyone is actually a stranger. So he is probably just starting to realise that he hasn't met this person before. So totally normal. My DS is just coming up to 3 months and I have seen it a little bit lately. I would just let him be around someone first before handing him over and not make too big a deal out of it otherwise he might feel anxiety on your part and reinforce any feelings of anxiety he has about the person.
Babies are so interesting arent they?!?