I would suggest that you consult with your vet.
My OTTB was a hard keeper when I first got him. At the time, my vet recommended Safe Choice as the grain at the barn was not the best. She also recommended 8 ounces of aloe vera juice once a day just in case he had ulcers. After a few months, there was a definite change in his appearance.
My vet also recommended feeding him more hay than he could consume. It’s amazing what hay can do for a horse.
I have since switched my horse to a blend of Cavalor feed (Strucomix Senior, Strucomix Original and Perfomix). He’s been on that for almost one year. He gets three pounds twice a day and anywhere from six to eight flakes of hay a day plus grass in turn out.
Cool Calories is a good weight gain supplement and SmartGain works well too. The Thoroughbred aftercare program I volunteer for is using that on one of their horses with excellent results.