what do i do

i have a boyfriend and he's kinda cute, and really nice. but he never talks to me anymore and todya he didn't even say goodbye just stood there for two seconds then walked outside and when i went out there he was gone. this other guy really likes me and asked me out but i told him i had a boyfreind. he still really likes me and i like him. i don't know how to break up with my current boyfreind and would it make him really mad if i was to go out with another guy right after we broke up.