Poor kitty.
Hope you've had him checked for parasites since the sudden weight gain. Colloidal Silver is pretty harmless and worth a try, but be careful with iodine. If you try the Iodine be sure to monitor your cat for any negative changes in his overall behavior.
You also might try some homeopathic medicines since they are the safest meds for pets. There are several posts on the forum about Paralixer by King Bio. They make other meds to (www.kingbio.com/naturalpet/productlisting.html) including an Appetite Control.
I have recently been using one of the King Bio "SafeCareOTC: Mind & Body" products on my cat for nervousness. Its from the human line but since its homeopathic all their products are safe for animals too. So if none of the pet products seem right then you might try to approach it from the mental/emotional side. Why is your cat fat: is he sluggish and disinterested in things and so not getting enough exercise? is he lonely and needy and turning to food? is he a lush and can't resist food, petting, toys? etc.