Something to consider first would be to cut his feedings to every 6 days. If he is sedated and laying around the day after eating, you know it was a good meal size. If they are up and about the next day hunting and prowling, try a bigger size. You can try a larger rat. By medium size, I'm assuming a 6 week old rat, about 150 grams.

BP's are oppurtunistic feeders and will overeat if given the chance. Just make sure to use common sense when feeding and I think you'll be golden.

A medium rat seems more than enough for a BP at 1050 grams, but something else to consider is he a chunky butt? I have some snakes that will eat a rat, and go cruising for more, I can give them another one, and they are still looking famished. Those are just my piggie eaters and take full advantage of what they are offered.