Well trust me when I say if something stupid is going to happen, it's usually to me
Honestly while it was un-set for your BF to make accusations regarding the person who he THOUGHT had text messaged you, I've got to say that even if the message in question wasn't/isn't from the EX that you saw out last week.. it was still from another GUY who isn't him (your BF) it's still from a Guy who has an interest in you other than friends.. and so yeah I can see why your BF was pissed about it.
While you can't stop someone from text messaging/calling you, you CAN let them know in no uncertain terms that you're NOT interested in them and not interested in them text messaging/calling you to flirt, know what I mean?
Sounds like your BF has had some issues with not trusting you.. so I guess him freaking out couldn't have been to suprising...