It sounds like you've tried many things. You might want to give this a try, I've heard good things about it:
http://www.preciouscat.com/cat-attract/I guess one of the symptoms of hyperthyroid is increased urination. Perhaps that is a connection? Have you considered getting that radioiodine treatment?
Does he ever use a litterbox when he urinates? You say that he urinates on the side of the box, are you using exclusively hooded boxes? If so, definitely try some non-hooded ones. Also, how large are the boxes you are using? Big male bengals need BIG boxes. Many people resort to making their own because the commercial ones sold in pet stores are too small by 50% or more. I bought the biggest ones I can find, and say they are barely big enough for my females, which are much smaller cats than your big guy, judging from the picture.