I think working him will develop muscle and help fill in his topline even if he doesn't gain weight.
I hope so. :~)
Oh - and on free lunging, I know the whole story ;~) and agree, for discipline... But I'm not lunging for discipline so much as just exercise, get him breathing and sweating a little. If he wanted to run the whole pasture, I really wouldn't care as long as he kept moving. He moves better loose, in the turns, which I just find interesting and good to know.