Normally when it comes to the horse world if ignorance was energy, it would be a nuclear powerhouse but, I am so blown away by the great advice you guys gave! I would suggest when you clean the hooves pay attention to any signs of laminitis. Normal for most horseman. Anytime you get a rapid gain it can be tempting other issues. These gains are not unreasonable in my opinion but, better safe than sorry. While you have a low starch approach, this will help keep the lactic acid down but I would still be using a probiotic with HUGE amounts of CFU's. I like the new Equiotic. It is equine sourced so, unlike others it doesn't all die off.
With the erratic weather, and the dynamic metabolic energy going through your horse, you need to watch very carefully. He looks great. High starch is your enemy. I like Uckele Cocoa Soya for fat top dressing.
The horse looks great. If I could make a suggestion. If he was off the track, there is a real great chance that he might have a compromised GI track. I have had awesome luck in putting weight on rescue TB's with Equiotic. Weight gain supplements are for doing one thing,....putting weight on. But, if you remove the toxins or reduce the toxins in the GI the weight comes on as was intended.
Just an opinion. Look at the cost of the Smartpack and compare it to Equiotic daily. You will get more for your buck.