I'd suggest trying different lubes. It may sound crazy, but some lubes can make a real difference. I find that for myself, any that have a mint base, or similar will prevent me from getting off. And depending on how "loose" it makes it feel, I might not get off. My old standard is petroleum jelly....but spit is a great natural lube. You might even try, if you're both piggish enough, having other guys fuck him first, putting their loads in him, then fuck him. I have known guys who can't resist shooting, and shooting quick if they know that their cock is going into an ass filled with other guys cum.