
Maybe his glands are just starting to block up? Unless you've changed something to try and combat this problem since they last blocked, they'll probably do it again, and without being emptied regularly (i.e. when the dogs goes to the toilet) they can fill up pretty fast.
Other things that come to mind would be abdominal discomfort (would translate to general discomfort, unwillingness to sit/lie down etc), parasites - does he need worming? - and an allergic reaction as Beckyjess suggested. His hips might be aggravating him as well. Difficult to say from an online thread of course! If his appetite doesn't pick up I'd get him to the vet.