
Before you rush out and buy anything like glucosamine (and most good dog food contains this in any case), I think you should wait to see your vet - unfortunately given the breed, I've gone to this potentially being something more serious than 'just' arthritis (unless you've had this confirmed?). I'd let my vet palpate, and if necessary x-ray before you start any supplementing.
Lots swear by glucosamine, but recent research has suggested it's not the 'cure-all' it's said to be. It may be he has to be on something a bit stronger when, assuming this is arthritis, it flares. It's bound to be worse during damp cold weather than in the dry warm summer days. Exercise of course because arthritic people need to be kept moving, but sensibly. And you might source a dog hydrotherapy pool because this provides gentle exercise for arthritic joints.