Sounds like he has a really nasty tapeworm "load". I would worm him again with Ivermectin Gold at 4 weeks, and again 4 weeks later. After that I think it would be safe to go to a normal worming schedule. Make sure you clean up the manure so he doesn't re-infect himself.
I once bought a filly - 10 months old - that had never been wormed and was in a "bad" setting. It was October, and because of her worm load, she had no winter hair...which in Wisconsin is not a good thing. I wormed her with ivermectin but just a half dose because a full dose would have the potential to release all of them, and that could kill her. She passed very little manure but it was LOADED with bots. I had never seen anything like it. 2 weeks later I gave her a half dose again...with the same result. Another 2 weeks and she received another half dose. This time there were no bots. I body clipped the "pig hair" she had for a coat and her winter hair then grew in.