He told me while I was out visiting my parents it would be okay if I had sex with his best-friend. & this weekend I did. But now I don't know if I should tell him because what if he gets mad anyway.

I have to ask, what sort of relationship are you in that one of you goes out for the weekend and the other says it's okay to sleep with someone else?? Are you in an open relationship where this is something that you both do regularly?

Without trust and honesty, there can be no relationship. I wouldn't hide it from him. If you were concerned with him becoming angry, the time to worry about it was BEFORE you slept with his best friend. And what sort of friend is this that he would sleep with his best friend's girlfriend?

Would you care if he slept with your friend while you were gone?

If you have a open relationship where either is free to sleep with anyone they want ( some do) then it is OK,

In most relationships it would not be acceptable. How did this even come up, why did you sleep with him, do you normally just have sex for fun, without any emotional ties?

Uhm, darlin, that is not a good idea to even think about sleeping with his best friend in the first place. Even though he says he won't care, he will deep down inside. My advice to you is that you need to tell him about it and face the music. Honesty is the key.