You have to decide what is best for you. Keeping your guy friend or respecting your boyfriends wishes. You have to honestly think about why your boyfriend doesn't want you hanging with your friend. Is it because he feels your friend is a bad influence? Or is it because your boyfriend wants to control you? How does he feel about your other friends? If he tries to limit other friendships as well, lose him. If it is just this one friend....You will have to decide which one you want to be true to.
I'm hanging out with a guy friend tomorrow who my boyfriend told me not to hang out with.. I feel guilty but he will be so mad if I tell him?
That is a choice only you can make, because only you pay the consequence for the choices you make. I am assuming you already are aware that deceiving your boyfriend results in consequences .. And that you don't feel cultivating trust is an important factor in this relationship. If you feel it's better to lie and go behind his back ..and that is what you are building your relationship on, then go for it. He won't be around long, and you can hang with any male friend you want without feeling guilty.