Maybe I need better pictures due to his dark color concealing....something, but I see a big hay/grass belly, probably from being on pasture all summer, not a horse with an alarming amount of body fat. He needs more muscle (work), for sure, but his body fat doesn't appear to be alarming. He certainly could lose weight but 'such a chub'? That big hay belly can reduce if he's fed less bulk feed, being off pasture should help. Timothy/orchard grass might be alright for him, but given how small he is he may not need 15 lbs of it. What's his weight and girth measurement?