Maybe if his parents dance there as well you can have a word with them, they are most likely to be the best people to talk to him about what is and what is not acceptable and communicate it in a way he understands, most certainly you have to do something if you are risking your back... as you've said your only 18 (nearly) and don't want to f***it up.

Have to say I'm a bad follow in that I chatter all the time (just ask me to be quiet) but would rather dance with someone talking about Eurovision than spending a whole song arm jiving with an old old bloke who tells me he doesn't get out very much and that this is his main social event of the year (happens a fair bit at some of my local freestyles, but also can't bring myself to refuse them a dance after all its only 3 minutes of my life)