Contacting your vet is a good start. But actually a lot of vets don't know a lot about nutrition, so my suggestion would be to talk w a equine nutritionist.
I myself have had good luck putting "skinnies" on a senior feed to start (yes even if they are younger), as it has a higher fat content, as well as a broader range of vitamins n minerals that most thin horses have been lacking.
You could increase his oil, and also adding ground flax to the diet would be helpful as well. Im a huge fan of Beetpulp too, so definitely keep him on that
Just be careful when increasing feed, do it gradually so not to shock their system. Same goes for switching from one feed to another, a gradual change is best.
Horses are like chips...you can never just have one