It looks as if he has been moulting Melon. They can look quite tatty when they do this. and it seems to get worse as they get older.
As long as the fur is not completely gone and is not red and itchy I would not be too concerned. If he seems itchy it may be best to have him checked for mites at the vet and get some treatment if it is this. As their hormones decline with age males certainly get thinner fur.
He looks gorgeous though