Complain to the club. In the very likely event that the club management will take his side, you could try complaining to the
SIA, which has a strict code of practice. However, I believe membership/qualification is only mandatory for door personnel working for private, contracted security companies, and not for in-house security (ie those employed directly by the club).
You're unlikely to get much success either way, though, I'm sorry to say. Clearly, if he's employed by the club, they're going to take his side. And whilst the SIA is great in theory, few complaints against security staff are successful for the complainant. They're a bit like police, in that they just seem to justify any violent action by saying that the complainant was causing trouble and of course he's going to say he was doing nothing wrong.
It's never been any different. I can remember my brother-in-law bragging about such treatment of clubbers when he was a bouncer in the late 1970s - only then, nobody seemed to complain, probably because there was no-one to complain to back then.
It's one of the reasons I've never been drawn to clubs. You can have a far more pleasant evening in a good pub with a decent live band - try it.