It could be a few things one of which is Cushings disease. There is an article on the WIKI about that.
Older hams often suffer hair loss. You can try adding some debittered brewers yeast (from Holland & Barratt) to either some porridge or sprinkled on his food and this often helps. Also a drop of codliver oil once a week but some hamsters wont eat this and it is a bit smelly. Maybe spoil him with a little baby food, chicken, pasta and porridge. He is still the same hammy just without his coat on