Not all situations allow for oil to work well, not all horses will eat oil (especially if only getting a ration balancer).
Not all horses like 100% prilled fats.
30-40-50% fat supplements can, and have, worked very well for a lot of horses.
Everyone has a favorite. It’s really a matter of picking something and trying it, hopefully being able to purchase small quantities to start, or know someone who would love to take a larger amount off your hand at cost or even a bit less.
I’ve used Max-E-Glo successfully in the past. Purina Amplify, Buckeye (I think?) Ultimate Finish (in 3 different fat %s), Triple Crown Essential Omega Blend (an oil), ProElite Omega Advantage (lower % fat), and many others, all can work very well. Cocosoya oil is another that is pretty palatable, and they have a solid form as well.
Plain vegetable oil is the cheapest for sure, but not always doable.
The alternative, depending on just how much weight he needs, is to give him a regular feed for a month or 3, get him to good weight, then transition back to a ration balancer if you can.
Or, add alfalfa pellets to his ration balancer.
Calories don’t have to come from fat, though the higher the fat %, the lower the volume for the same calories.