They could get done for causing an affray. I mean...you DON'T turn up at a club wearing trainers, for goodness' sake, so your bf shouldnt've argued about that one.
I hope there IS CCTV footage, because it seems unusual for a doorman to kick someoine in the way you said, for no other reason than that your bf and his pals wouldn't take no for an answer. If the dooman gets done for assault, he'll probably lose his job at the least. If your bf and mates get the same, it depends on how they plead, and what evidence there is.