If you want to add electrolytes, add typical electrolytes without sugar to his feed. Or try low sugar Gatoraid (which I drink all the time). I add electrolytes every day to my horse’s feed.
For a nice snack, try carrots in carrot stretches. My horse gets this as a reward after work and he totally drops because he knows it’s a reward and then licks my hands profusely afterwards. If I move my hands, he “lips” them back. For whatever reason, this is a very relaxing moment for him and I let him take his time licking my hands. I pet his face and forelock, too. I often hand graze him even for 10 minutes after a ride, because the grass around the arena is better that the grass in his pasture at this time of year. And, I know where the clover patches are.
In the summer, he has a mild fly allergy and I break out the human back scratcher after the ride. He loooooooooooves it. I realized recently that he loves hard grooming so much (scratching of his butt and shoulders) that I wore the teeth off his Epona flower shedder. It’s totally smooth!!! His new one came today and what a difference.
Bottom line, find what your horse considers a real reward and likes!
You could also consider, if he likes carrots, grating them into a bit of feed feed as a healthy snack. Or apples. Some horses really respond to peppermints and while all sugar, are tasty enough that a couple crushed in some feed might be a treat.