It comes down to the whole 'reasonable force' thing. The bouncers I've known and worked with have been properly trained and qualified. They will tell you that their job is to ensure the safety and welfare of the customers, and to ensure that they (the customers) have an enjoyable time at the venue. This can't happen in an atmosphere of violence and fear. Any bouncer that tells you otherwise isn't fit to be in the job.
Anyone with drugs, who's drunk or causing any sort of trouble should be asked politely to either cease the offending behaviour or leave the venue. If they persist, then at least two bouncers will take steps to eject or restrain them. I have seen bouncers using varying amounts of force to do this, sometimes in relatively secluded areas (this was at a music festival), but at no time have I witnessed any of them 'put the boot in' as it were. As enigma pointed out, they are advised not to deal with people single-handed, for physical and legal safety.
There are those who go over the top and get carried away. The job does hold a certain amount of power and some will make the most of it. Again, any establishment that employs people of that ilk should either deal with them (by dismissal or 'training') or shouldn't be patronised.
If your bf feels he wasn't being overly agressive and that he really was being unfairly treated, then he should complain to management. In the venue I work at sometimes, if security are reported as being overzealous then it's investigated and dealt with. Also, all staff are expected to report such behavour. They really do want people to have a good time there and take great pains to keep security low-key but still effective. That's a very fine line to tread and they occasionally fall off, but they're working very hard on it.