Thanks for your replys everyone - Jimmy was at the vets recently because of a problem with his throat (which is now sorted) and they checked him out and said he was ok. They're not sure why he does it.
The vets keep a good eye on him because he nearly died when he was a kitten, due to a hole in his bladder, which caused infection to his insides. Anyway, he managed to pull through (he's got a lot of spirit
) and is now doing fine.
Sometimes when Jimmy goes into the litter tray, he just acts as though he's weeing normally (no straining or anything) then when he gets out, there's barely anything there.
There is only one litter tray, but both cats go out at night and do any pooing outside - the tray is there for them when they're in during the day and I clean it out as soon as I know there's something in it.
Maybe he just likes going in there for some reason
he has always been a little bit behind, bless him