You might need to take him to a groomer to have extra hair trimmed away from the rear end. For the rest of the body brush on a regular basis or take a dampened wash cloth and wipe him down. Frequent grooming works better than a marathon session every few months. Experiment witih different types of brushes so you find one that feels good to him. I use a flea comb to remove the undercoat. Reward with a treat a the end of a session. Hope you can get this accomplished.
If he is older and getting arthritis, you might want to try ARTHRO-IONX. If it hurts him to move and twist maybe this might help him to do so more comfortably and could assume some of the grooming activity.