This is absolutely astounding. I thought it was a photograph at first site.
This is not critique on the drawing itself, but those brown blotches on his tail are from his.. Erm, pottying. It takes some good scrubbing with a soft-bristled toothbrush to get it completely out. That's why I started using blankets instead of bedding. Easier to keep their tails clean. There is a shampoo called Squeaky Clean that is very good for use on rats. In fact, my big guy Willard loves it. And make sure the water isn't too cold. It should be quite warm. Warmer than you'd think. Rats have a very high body temperature and chill very quickly. Your fella looks very healthy. His build is impressive. Especially his head.