[souffle]

I think you will find that this is a natural ageing process in dwarfs. The organs become less efficient and they lose weight and muscle mass and sometimes experience dry skin and some fur loss. If the kidney function isn't good they can also wee a lot more.
It is a fine balance between giving more protein and giving too much which will strain the kidneys and liver further trying to process it.
Your hammy is a wonderful age so you must have cared for him very well. Make sure he is in a reasonably warm room and you can supplement with a little tofu, pasta, egg etc but keep up the hard dry food to keep the teeth in good trim.