Technically, there's no actual adipose tissue in the brain. Fat in the brain is a lethal condition called Tay-Sachs disease. However, he was fat during childhood, puberty, and teenage years. We know fat clogs arteries, reduces lung capacity, fucks up hormonal activity. I think his brain is now suffering from the lack of oxygen, blood, and testosterone in those critical stages of development, which is why he's now angry, retarded, and moody. I don't know if it's reversible, I couldn't find anything in the literature.