If he is roaring that badly, I'd be questioning his quality of life honestly.
Most of those problems can't be fixed with suppliments, although keeping him on a low sugar diet MIGHT help with gnat and such somewhat, if that is the cause of the sweet itch. Buying him a fly sheet to wear that covers his neck and tail, and adding apple cider vinigar to his diet might help also. If he has a food allergy or sensativity causing it though, you'd just have to experiement to find what was wrong. Do you put anything on his sores/rash to make him more comfortable? We've only had one case of sweet itch on our farm before, and it was due to the gnats. We were told to apply baby oil to the base of his mane and tail, and to cover him with a fly sheet. We were also told to apply an anti-itch cream to his bad spots, and to use baby pwder.
The hock clicking could be from many different things. It could be nothing, or it could be artheritis setting in. How old is he?
I've never used this, but a lot of people on HF use MSM suppliments. Maybe that wold work for your boy? Its an all-around suppliment that helps with joints, respiration, and skin problems according to the website. MSM for Horses - Horse Joint Supplements from SmartPak Equine