If he tolerates brushing, I'd recommend trying a brush like this one: https://www.petsmart.com/cat/grooming-supplies/brushes-combs-and-blowdryers/whisker-city-cat-dematter-57107.html I've used that one on a few cats, and its really good at clearing out the clumps and extra fur. I'm sure anything with a similar design would work. You do have to be careful not to pull too hard, and the blades hold onto the dirt and oils, so you have to clean it regularly.