It's a bit of sticky thing, really, Ilysa.
Technically, he can come over without a visa and stay for 6 months. However...
If he comes to the UK expecting to say he's staying for three months, the immigration officer will want him to be able to prove:
-- that he can support himself for the length of time he will be here (and him saying you will be supporting him probably won't be a good idea. He'll need to show that he has the funds to do so himself.)
-- that he does not intend to overstay the time he says he's staying. So they'll want to see return tickets, a letter from an employer stating they expect him to return, a letter from a landlord saying his lease is still in effect, these sorts of things.
If he's been stopped and questioned before, immigration may have marked his passport somehow to show that they were suspicious. It may not be right, but this may flag up earlier problems and lead to more questions.
He might not want to mention you are his girlfriend either. Best thing is to say is that he is visiting a friend for an extended holiday. And then make sure he isn't carrying anything that might prove otherwise such as loveletters, nighties, etc....
Best of luck!
