Firstly, please don't bump posts after only half an hour or so. You're asking quite a specialized question so it's unlikely you'll get a response straight away.
With regards to withdrawing a reference, this would be highly unusual, and I think it would be best to contact UCAS directly to see if this could happen and if a uni could withdraw their offer in this circumstance. My feeling is that if he was able to withdraw it as it was no longer accurate, then it might be possible for unis to reconsider their offers, but really this is something you'll need UCAS to clarify.
Chances are it's just an empty threat, as this would be pretty extreme action to take unless there's actually more to the situation than "not trying very hard".
A bigger issue would be if you were no longer allowed to sit your final exams. Unconditional offers are usually still dependent on you taking all the exams you said you would in your original application.
It's also worth bearing in mind at uni you'll need to be very self motivated- there won't be anyone checking up on you or nagging you to do the work. If you get into bad habits at this point in sixth form, it can be hard to get back into good habits in September. Also, even if you've got an unconditional offer, future employers might look at your A-level results, so it's in your interests to get the best results you can.