I need some advice? Calling all ladies?

Okay, so first of all, I am young (16) and I am a virgin. But, my boyfriend and I like to do stuff other than sex. Sometimes he can insert his second smallest finger without much pain at all (he does have really big hands), and sometimes it causes me pain so I have to make him stop. I know this probably has to do with arousal and relaxation and me being a virgin, but does anyone know how I can fully relax my PC muscles (vaginal muscles) during this, and not worry if it will hurt at all? Or an exercises I could do to make me a little looser?