One small rat every week should be fine. Small meaning ~45-85 grams, but even just ~50 grams is fine. His weight is good and at his age he doesn't need to do any growing. You just need to feed him enough to maintain his size. There's really no reason to feed him medium rats or larger unless you just want to spend more. Quite a few breeders and people on this forum only give their breeding females one small rat eat week. If you over feed you're more likely to cause him to refuse food.
Can he eat bigger meals? Yes. Does he need to? No.
If you use the 10-15% body weight rule than that would be a rat that weighs 100 to 150 gram which would be a medium rat. Also he did not want to maintain his weigh, he wanted the snake to gain weight!