Most Helpful Guy
just start with small chatter and keep it going or tell a story something he can relate to. Quiet guys can be super chatty just not in public. Its a funny fact about lots of introverts its not so much they dont like to talk its that they dont like to talk and opress or feel liek they are offending others. Part of the reason in some cases introverts make great bosses as they dont speak before they think.
Most Helpful Girl
I do the exact same thing when I'm alone with friends/strangers/almost anyone. Its probably a part of his personality. For example, I feel comfortable in big groups as I don't feel pressured to talk ALL THE TIME, I can just talk when I have something to say. When I'm alone, it stresses me out because as a naturally more shy/introverted person if I don't have something I really want to say, I'm bad at small talk. Also being around my friends gives me confidence and makes me feel like I belong.
This might not be the case for him but it very well might be and all I will say is don't give up hope or feel like he isn't interested/etc, if you just hang in their and try and catch him alone a few times so he gets more familiar with the situation. Also try and make more conversation so he doesn't feel pressured.