Mouse pinkies for a nine month old BP are WAY too small; this is why your snake is underweight. It's emaciated.

I feed my snakes based on body weight, and feed a prey item based off of 13% of their body weight. At 270 grams, your snake should be eating a rat between 30-35 grams, which is around a weanling rat. You could feed one of those every week. Try rubbing the rat with a mouse before offering it, that's how I switched my snake over and it was super easy.