sorry but if this is not your horse , you shouldn't be touching him unless you have permission.
You could suggest things to the owner , I would be thinking he needs a wormer and a change in diet , perhaps teeth as well.
Opps just re checked your last post. Yep give him a worm, make sure he is getting some mineral/vit supplement and mayby start adding a small amount of oil to his diet. But a grooming session whenever you see him will help as will - I love the jelly combs, both my horses love it!!!
Can you post some pics?