Thanks for the responses, people. I think the first thing to do is weigh him and see what he comes in at and go from there.

I will also bump up the meals. I'll take him to the vet next week if he's still looking dodgy.

Incidently, I questioned (on this forum) a few weeks ago whether or not 1 large mouse per week was sufficient. Had a number of responses, several of whihc were "He should be eating rats at that age", so I guess it's possible I haven't been feeding enough. The main concern however (as noted by Casey) is the sudden weightloss as I hadn't noticed this until recently.

Off to buy some scales - will keep you all posted and thanks again!