I been dating this guy for a year.and he doesnt really like it when i hang out wih one of my guy friends.why?

i have this guy friend which we been friends longer than i known my boyfriend.why is it that my boyfriend wants me to be striaght up with him about everything. but yet when i tell him that im out hanging around with my guy friend he gets mad. my boyfriend can hang out with him, but then why does he make it a big deal when im with him? so yet its hard for me to tell my boyriend that im going to hang out with him . so what should i do??