Obviously a big problem. He has overheating issues when sober? Any diagnosed conditions?
Serotonin helps control body temperature, when you release massive amounts of it (like on MDMA), your body has a hard time dealing with these fluctuations and body temperature can raise or drop significantly. At a rave this can be a large problem, especially when the room your in is over 100 degrees by itself!
Fortunately, most raves and clubs (at least, around here.) have a "smokers section" that is outside. I would stay near there the whole time, obviously still enjoy yourself but keep some escape routes on hand so if you DO get too hot you can just go chill outside for a bit until you're cooled down. Or try and find a rave that allows you to enter and leave as you please, so you can go for a stroll around the block if you must.
It shouldn't be a HUGE worry unless he has some kind of condition, but obviously being safer is better than sorry