And then I went and forgot to answer your question from before!
I feed him "jumbo" mice, the biggest I can find are at the local reptile store and they are
huge so no need to move up to rats just yet! The last one was obviously pregnant, that really was one gigantic mouse.
For the past couple of years I've only fed him every two weeks (bar the odd missed meal when he's in 'blue' or 'randy' mode), because as I mentioned before I've been worried that he is overweight. I think I'll put it up to one jumbo every 10 days, he's obviously just meant to be big and his diet may as well go out the window