I recommend broodmare mixes from your local grain store, they are high in fats and conditioners. I use it for my TBs who I retrain off the track for eventing, helps them fill out.
Also, it could be a muscle issue, are the horses ribs easily seen?
After worming there should be an improvement.
My 12yo gelding has been in 'very' light work due to discovering he has a respitory prob (developed as a youngster) and since then his muscle is peeling off, his rump is losing its roundness, same with his back etc, his topline is melting because of not enough work. Yet, his ribs are not visible. Poor boy looks older than he is but I am unable to work his muscles as often or ride for long because of his breathing.
Anyway, a little work might help the horse build up too, the good fats may end up in the wrong places(low belly etc)
Maybe ask your vet to check his old injury and decide whether he would be capable of exercising