Most Helpful Girl
It depends on their intentions adding certain girls on Facebook.
I don't have enough information to be upset with him, and I'd jump into conclusions. Being in a relationship with my boyfriend would need trust in each other, so I'd trust him until he gives me a reason not to.
Plus, just because they don't have mutual friends doesn't mean he's sending/accepting friend requests from a random girl. They might've met in class or something.