@OP, that rat is fine, but depending on how he responds as far as taking it or not you may want to drop down in size a bit and just up a feeding schedule to say 5 days versus weekly. If he's underfed he may not want to take something that size but hopefully he does. Let us know.
You can't always go by the 10-15% of the body weight rule. You strictly go by the biggest part of the snake.