I would not want to see him lose any more weight...the beginnings of a concave flank I saw, one of the first places to show when more is needed or to much is fed.
He's cute and he looks OK currently.
If he is standing in his pasture in that picture, then it is gone, near no morsel left for him to eat.
Yes, he probably is hungry, hence under every fence pushing for anything he can grab.
What you are feeding him with what he stole under fence-lines was
Now that the edges reachable are also gone, eaten away he will not keep his appearance for long is my thought.
No longer will stolen morsels be enough...the morsels are all gone that sustained him.
You need to up the food fed, such as forage...its naturally gone and you as caretaker must give what nature no longer provides. Do you have availability of hay cubes?
Alfalfa, T&A, brome grasses...cubes at least give some long stem fiber and forage where pellets are so processed...it works different in the gut.
A few pounds a day in addition to what he currently eats to replace what he no longer can find for himself.. Always
soak cubes before feeding so the choking hazard is reduced significantly, not a bad idea for pellets either as they swell when hitting moisture such as saliva.
You truly don't want a blockage in the throat, it is a dire emergent situation if the horse can't clear it!
If the other horses on the farm look poorly, it is the warning of where your horse will be shortly... I would be looking for a new place already..