All of them. He would win against all of them. And none would last out of round 1, and less than 5 would probably make past 1 minute.
This might seem strange to someone who has never trained. But walk into a boxing gym, train for a week, then fight the club pro.. It'll be easy to understand there is no lucky shot you could ever land to beat him in the ring. Now imagine the same on a bjj mat.. How would you ever submit or even just survive against a black belt? You couldn't. But these are fair fights with rules, anyone with a gun in their hand has the potential to kill anyone in a split second.
But in competition, no untrained fighter would beat someone like you described.
I'm talking "very experienced" and assuming this means he's had more than a few mma fights and probably fighting at the pro level, above average skills for a pro fighter etc. is this what the OP meant?
I'm going to disagree with this, but purely based upon my experiences. I have minimal serious muay thai training and very rarely work my stand up consistently. I've sparred with a few pro boxers that have passed through a gym I was training at. Now I don't know a ton about boxing but I held my own, fought well and dropped one(the crowning moment of my striking career). I don't think it's impossible to land that one clean shot, granted I've been known to have a hard head and be able to take a lot of punishment combined with pretty heavy hands, not to mention I stand southpaw. But if I can drop a boxer in a pure boxing match I don't think it's impossible to think someone couldn't get lucky once.
I don't think it's likely, but anybody can be hit.