Appreciate the candor.
Well, in my opinion your boyfriend (although valid in his feelings), was extremely immature about the whole thing. Instead of just saying (and still not saying) "I'm not comfortable with you messaging a rapper" or "I'm not OK with you smoking pot again like when we were teenagers" he's giving you constant hypothetical questions and acting out/finding fault where he can.
You're allowed to feel blindsided, and he's allowed to be jealous. But he shouldn't feel warranted being mean about it after helping you construct the messages you sent to the rapper, if he didn't want it to go further he should have said something.
I'm not so sure about this next point since he infers you never gave him the chance to go with you, but you claim you asked "Can I bring you?"
But on his side he's probably feeling pain for a few reasons too.
He sounds like he didn't expect you to actually want to do "anything" with a "celebrity" hang out or otherwise. In my opinion it sounds like other than it being a funny novelty, that it takes a different "type" of person to actually want to do something like partying with a rapper in his eyes.
He thought smoking pot/drugs were something you had both moved on from since being teenagers.
If those are real issues he has they're just as valid.
Real talk to both of you. Get off of Reddit and sit down and talk about the real issues on your minds and tell the truth.