He'll be fine taking a meal that's a bit too small. They don't have to be huge for them to get a good meal, and having a meal that's too small every once in a while isn't going to hurt anything. Also (if he's big enough) you could feed him 2 rat pups instead of 1. I still think he'd be fine with a smaller meal if a large one isn't available. My oldest snake was fed smaller meals a few times and he never stopped growing. I noticed no decline in weight gain or anything.