From personal experience I don't think its possible for a guy to come more then 2-3 A HOUR. After the 2nd or 3rd one, he will last a long long time and prob. pass the hour mark.
Having said that, I will testify that I have gone on a 2 day sex marathon and as far as I can remember came over 15-20 times. But at that point you don't care about coming, you just don't want you dick to fall off or start bleeding. lol. So cumming more then once or 2 times doesn't hurt, but your guys junk might start to get raw/sore. So please keep an eye out for your guy.
As far as helping him, the hornier he is after his first load is spent, the better the next on will be, and the next one(if your lucky). So get him really worked up and hope for the best