I'm kind of lost...
Does he need weight or muscle tone?
Oil, because it is a "fat" will help with putting weight on the animals frame but will do nothing to put muscle on.
Muscle, you need to build muscle by work and training...same as a human...the fitter you want to become the more you slowly put in the effort to achieve harder and higher work loads.
You start at a slow rate and gradually add time duration and difficulty of the work load. You need to work the musles consistently to build "mass".
To build a hind end do trotting, slow and even, faster and slower, just faster, up and down inclines, backing...he needs to push from behind not pull himself around on his forehand to build a tush.
Transitions, different gaits and speeds during those gaits, gentle turns, serpentine and circles...what ever you do one direction you do the same the other direction to build equal muscle mass, stamina, co-ordination and balance.
I would not look to feed liquid oils myself, they do go rancid easily in warm weather, many horses do not like their food greasy & oily.
I would purchase a horse fat supplement in pellet form to top-dress the daily ration of feed.
There are many feed manufacturers out there that make a easily handled and safe to feed supplement.
If the horse/pony though is of good weight but lacks muscle, he just may need a better regiment of conditioning to be started.