I do not think that snake is too thin in any way. He looks to be just right to me. See how he has the fat deposits near the base of his tail? He is a good weight. However, I can NOT see how a 3ft BP would be only 450gms!! My '09 4 foot female is over 1900 grams (and she IS a wee bit on the thin side - no real fat deposit around the tail). I wonder if the scale wasn't set right or not calibrated right?? I would have guessed a 3ft snake looking like this to be in the ball park of 1000 grams - not 450. No way.
Side note - those "fat deposits" at the base of the tail are not the be-all and end-all of determining if your snake is an appropriate weight. But you wouldn't see deposits like that at all on a snake that was too thin.
An appropriate sized rat once a week should be just fine for this snake - especially if you're not breeding him/her. Even if he does skip a meal or two once in a while, he should grow and gain weight at an appropriate rate on that schedule. There is no need to rush getting weight on - especially since he's not under weight. He'll gain at the rate that's healthy by simply feeding him as you currently are.
Incidentally, overfeeding can cause a BP to actually go on a hunger strike. I would not necessarily recommend feeding any more frequently than once a week. Up the prey size if you feel its necessary, but I'd stick to once a week.![]()