We've been through the blood testing Lonecat has described with one of our cats (turns out the culprit is a chicken protein), she properly chews on her skin when she has something she shouldn't and licks her tummy until it's bald and red. Half an anti-histamine calms down the redness (was suggested by our vet). A vet might be able to suggest a few things before going down the blood testing route, have you tried feliway or switching food to see if it is an allergy?
Is he making himself bald and/or breaking the skin? Have you been to see your vet? Usually cats that excessively groom have a complex problem going on involving allergies and to some level stress (though they may not look stressed). How far we investigate depends on how bad the problem is and to a certain level how much money you have to spend. Identifying the allergies by blood testing and then using special diets and immunotherapy is hugely successful in cats, but runs to at least £100s.
He is a rescue cat and we have had him nearly a year. Friendly big beast but is forever licking himself. He won't really play with anything either - occasionally gives a cat toy or piece of string a cautious dab but then goes back to furious washing. Any thoughts? He is flead and worked regularly.