"The previous owners had him on IntegritT - it's a low sugar feed that helps prevent excitability in hot horses (he is an arab after all).
You might just let him live on grass, and supplement as needed with hay. My horses in Arizona don't get much green grass, but they live off of hay and hay pellets (the small ones). The pellets are because it is often too windy to feed them hay, and our hay can be pretty variable in quality.
Then keep an eye on him. One of the pure Arabians we had was simply a sweet horse. She could get excited, but she was always willing to work with her rider. She was green broke when my youngest learned to ride on her, and no problems.
Trooper is our "Steady Eddie", and he's 3/4 Arabian. The Appaloosa stallion was 50% Arabian, and a very dominant & aggressive horse. Not Trooper.
Mia was a purebred Arabian I had for 7 years before swapping her for Bandit, who is half Arabian. Mia...well, she helped create the "Arabian Mare" stereotype. We did all the spins and bolts I ever want to do in my life...but there also was not a mean bone in her body. My son has been on a horse two times in his life, and both times it was on Mia. That says a lot about her.
Bandit startles a lot, but he has never lost his mind doing it. Like Mia, I just accept the idea he'll get a little nervous at times - but in his case, I can usually figure out what it is. He doesn't bolt & he'll go around something if I can find him a little room. His instinctive reaction to something might be "Oh HELL NO!" at first, but he doesn't flee in panic and he is still willing to listen to suggestions. It is often possible to switch him to "I'll try...if we can take it slowly".
IOW, don't buy in to the idea that Arabians need special treatment. They are, IMHO, smart and alert. Training one can require a different approach than some other breeds. But they can also do this with a rider they trust:
I'd start your horse with grass (and hay if needed), maybe work up to hay pellets (predictable quality and easy to feed). I gave Mia a Mg supplement, and Bandit is on biotin, but that is it.
Good luck! Your horse has had FAR more training than any of mine. Enjoy!