Your message suggests that your guy only receives hay in *winter*, but is on straight pasture the rest of the year. This likely isn’t enough calories for your horse — especially an OTTB (high metabolism) who’s relatively young. As others have mentioned, not all pasture is the same: unless you’ve had your pasture tested, and do so regularly, you don’t *really* know if your horse is getting enough calories per day, and if what he’s taking in is nutritionally correct. I strongly suggest using a feed calculator to ensure he’s on the right track: there are several online (see below). Also, most feed companies have nutritionists on staff; call your feed manufacturer to have a free assessment. I breed, train, and show dressage horses, and have also adopted and retrained several OTTBs, and can tell you that after 35 years in the business, my equine nutritionist is the best resource I’ve got! My horses look fabulous and perform well, but only because I double check their rations / caloric needs every 6 weeks.
Using the info you posted (estimating weight at approx 1000lbs), I ran the calculation and it suggests +12 lbs forage per day plus 3.25lbs concentrate per day, for a minimum 13,800 calories per day. I don’t feed Purina to my horses, but I use their calculator because it’s quicker and simpler than many of the others. Triple Crown has one too, I believe.
No amount of supplements will make up for a deficit in calories. Start there — amount and quality of food. Call your vet or the feed maker’s nutritionist, and ask for advice / guidance to ensure you’re on the right track. Finally, get a weight tape and use it monthly to track gains or losses. As I tell my students: start with the simple, most obvious answer first and move forward from there.
Good luck, and good riding.