I wanna know why he keeps doing it..

in my class,whenever someone teases my boyfriend with another girl,he blushes and keeps smiling...it feels weird...i mean, does he really is shy about it or just playing around?but,he really seems shy when his buddies tease him with other girls..is he a player?should I trust him?he is not really a shy person...fyi,we never tell anybody that we are going out...so,his buddies don't know about it...but,why he had to be shy about it and smiles like a really bashful guy?i don't get it...