damn man i'd just tell him its too bad she wanted to suck your dick. an ass kicking is definately worth it if you hurt the kids feelings.....clearly he'll be crying himself to sleep. you might get beat up, but you can always get head from his g/f. screw it man.
if that were me i wouldn't let that shit phase me. don't avoid him, he'll feed on that shit and his friends will too. just act like you don't give a shit what he thinks. if he wants to beat you up, great. that shouldn't stop you from enjoying life. besides you said he's clearly gonna win. the worst thing you can do is try to fight and lose. then its like you're a dumb ass too. let him blind side you or do some pussy shit. it's hard to respect someone (even if they're your friend) when they hit someone who isn't trying to fight them.
and if he is confronting you don't say sorry....i'd just play it like i dunno why he's mad. if he brings up that his ex sucked you off and that's why he's mad...we'll he'll look like a dumb ass. and if you keep calm and explain to everyone else standing, but be talking to him "listen man it was one night, you were ex's and its over. grow up" he'll remain pissed off and everyone in the circle will be like "oh....well that makes sense. this kid who is mad is a faggot"
i had some kid last year in the dorms confuse me with someone else. i was sitting there talking to some kids in the hall drinking some beers. he sees me and starts yelling at me telling me i choked his girlfriend some night weeks ago. anyway, so i didn't move at all. this kid probably could've stomped me, especially cuz he was ragin'. but i just sat on the wall drinkin my beer and told him he was confused. all these other kids, his friends were taking his word and acting hard and saying shit like "man you like to choke girls, faggot". after about a minute of making light of their comments (them: you choke girls? me: yeah man all the time i'm about to go choke my cock upstairs) his friends ended up holding him back and restraining him in a room while i was sitting on the wall drinking a beer and laughing about how strange it was. its amazing how effective staying calm and civilized can be when dealing with irrational spaz's.
the kids on that floor told every girl they knew about me and how i choke girls (which was cool cuz then a ton of girls knew about me). so everytime news girls would kick it with us i'd get the standard "i heard you choked a girl, you don't seem like the type to do that." at the end of the night. i'd make a joke outa it some how and fuck 'em. that kid created a great ice breaker for so many of the girls i met in the dorms, i should've thanked that floor for handing me pussy like that.