Reminds me of "A Boy Called 'It'."
There wasn't, as far as I can recall, a "reason" they abused him, and there were other siblings in the home as well.
Evil, as far as I'm concerned, is the only "reason."
I hope this child hasn't reached the point of no return, with regards to his nutrition. I recently lost someone who, through a traumatic brain injury resulting in delirium, didn't/wouldn't eat more than a few bites a day. Even with IV nutrition, when he'd allow it, there comes a point called cachexia, where even if feeding is restarted, the body is unable to recover. 30-some odd pounds in a 10 year old is horrifying. Bless him.