I'd say it's much too late to reverse it now.
If you manage to get him down to the size where he can walk, he'll just get more food for himself.
And of course you can't relinquish your care completely, because he's overdependent on his father for everything.
That man is both mentally and physically ruined for the rest of his sorry life.
The reason? The father divorced, and when the "strict" mother kicked out the son, he wanted to show his virtues by granting him everything.
The truth is, the game was rigged from the start.