I don't think he looks skinny at all. People seem to have a fascination with fat horses. If you can slightly feel his ribs but they're not poking out he's doing just fine. Could use more muscle as someone already said but I think he looks pretty good to be honest. X
I don't see him as ribby really, either. He does look a little pot bellied, and under condition. When was he last wormed? It could be parasites...they will make a horse look pot bellied while steeling all their nutrients.