I would eliminate the sweet feed. Hay, minerals and salt only then if he needs extra for growth/work add something without molasses/sugar crap. Let him beg all he wants for the sugar. He's a kid - he doesn't know any better. :)
I would not ride him. At all. He's too young.
When he's hot and sweaty is his heart racing as well? Have you taken his temperature and respiratory rate? I would document those for the vet if you haven't already.
I would also check on him overnight. Hay and water only in his stall/paddock.
I hope it turns out to be a simple thing. Good luck.
I'll be interested to know what the vet thinks as well.