I agree with you feeding your Arab if that is possible.
My Arab is 26 and has always been a "difficult" keeper; now he's a hard keeper. He's dealt with mild gastric (stomach) ulcers for the last eight years but, thankfully hasn't had a flare-up for a couple years.
I always attributed all that to the fact he was a starving horse poster child when I rescued him over 19 years ago but, I am reading more and more about senior Arabs that are hard keepers.
I have run the gamut with tests, he gets physicals twice yearly, that include his teeth, of which he has four molars missing
He is ribby but not "turn this woman in" ribby. Like your Arab he is always fatter on spring/summer grass.
He is a really picky eater -- I mean picky - especially when it comes to hay. Once hay isn't "fresh" anymore, he doesn't want it
I'm glad I have mats on the barn floor because I have to rake up "the fluffies" for him. I'm ok with him not eating the stemmier stuff due to missing molars but, his desire for the prime flufflies is ridiculous and part of why he loses weight every Fall. You wouldn't believe how far we have driven for Prime Rib hay just for him and, sometimes he loves it, sometimes he doesn't.
Thankfully, I am recently fully retired so I can wake everyone up with a bit of hay or soaked cubes, then go back down an hour later to feed supplements and prepare everyone for turnout. SO, At the moment (because this won't last until winter), my Dollface Old Curmudgeon gets:
1) about a pound of well soaked timothy/alfalfa cubes in the AM. That keeps his protein/energy level up.
2) ~2 lbs of Triple Crown 30% (my gosh he LOVES that stuff!) (divided in 2 feedings)
3) 1/2 pound of equine rice bran (divided in 2 feedings)
I can't shove too much food at him because he just won't eat. He has 22 acres of pasture to wander around on so hunting him down to feed him 3X's/day, in front of three other horses, is not feasible. The two pounds of TC 30% gives him his required vitamins/minerals.
4) Omega-3 Horseshine.
5) Brewers Yeast
6) Extra Vitamin E
7) Arthritis meds
His coat and hooves both have a healthy shine and his little 13.3H self still maintains second-in-command in my herd of four. His little 26 yr old self doesn't have a bit of problem keeping the 16.1H TWH, that is a bully, in line.
While the vet would also like to see a few more pounds on my Arab, the fact that his eyes are bright, he is more full of mischief and business" than many half his age, the vet says not to worry.
My point to all that was, it could be your Arab is entering a era in his life where you're going to see fluctuation with his weight no matter what.
So the best you can do is to feed him in the most healthy way you can for who he is.
Keep his coat & hooves shiny with what you feed him, not what you spray on him - lol lol
Keep his energy level up with a bit more protein. You'll know when you're overloading him with protein because his urine will have a really strong ammonia odor to it. Horses dump the protein they don't need thru their urine, thus the stronger-than-normal ammonia smell.
I hope this helps some