Yep, emailed my tutor for my feline psych course - she says it sounds like he's having problems with his teeth or gums. He's prone to gingivitis too, so it's possible.
Have called the vet and he's ringing me back after he's finished surgery. I'm scared to take LJ in for work on his teeth though, because he's so underweight and only just started his fortekor.
Thanks Bazsmum.