Aww glad to hear he is doing well!! It is pretty awesome that you kept him btw
1. A small rat (around 100-120 grams) is perfect for him for the rest of his life.
2. Most people weigh their snakes on "empty". Right after they took a huge dump.
3. He shouldn't really get too much bigger unless he is actually a she (quite possible). Most males usually stay under 1,000 grams but it is not un-common for them to get as large as females.