Greetings Kitty24,
First thing I would do is ensure he is getting the thyroid problems treated, it will have a massive impact on his weight gaining and his diabetes. If memory serves me right the drug used is called T3 (chemical name is triiodothyronine), it basically replaces the hormones generated by the thyroid. Higher levels of T3 should cause fat loss in and of itself. For the diabetes side of things trying to keep the sugar levels in the normal range is best, since excess sugars are usually stored as fat. Other than that lots of exercise!
Hope that helps,
Frankie