I would turn him out 24/7, only bringing him in to eat and in bad weather. Movement is very important for arthritic horses, and it will help him keep some of the weight down. Does he have a friend to be turned out with? That should help encourage some movement.
I would also give him less grain, or just plain oats with some applesauce or a dash of oil to make his supplements stick.
Or, you might think about a ration balancer, like Triple Crown's 30% supplement or Purina Enrich 32. You only give 1 lb for full nutrition. Your boy would be getting better nutrients & vitamins, but less calories. Your joint supplements should stick to it fine too.