I dont know what to say about being 10 year old and 900 grams but I'll tell you experience.
I bought a VPI Axanthic, he was 1year and 3months old at the time. He looked VERY skinny to me when I weighted him. I was very shocked he was just barely 300g! He had been feed mice his hole life. 1 a week, from small to adult in the last few months according to his feeding record that came with him.
The first time I fed him, It was a very small, small rat. He took a while, but he ended up taking it. Probably very hungry.... I feed him a small rat every week. He has been gaining a lot of weight. He still looks thin, but much better. He is closing 700g now. Ive had him for about 3 almost 4 months now. Thats more than twice his original weight in 3 months.
Im not saying that is the same with your case, but maybe... just maybe he is being realy under fed. Do you know what he feeds the snake?