Young horses should have as close to 24/7 turnout as possible, to help bones and tendons grow strong. They need movement to harden and develop properly.
I see no reason to mix sweet feed and complete feed. I do not understand why people do it... I would pick one feed and stick with that. If he's getting less than the recommended amount of feed for his size and age, then he's not getting enough vitamins/minerals. You'll need to add a vitamin/mineral supplement to his diet, or switch to a different feed that you can feed less of for his weight/age.
I prefer to use no feed at all. I give my horses free choice grass hay, as much grazing as they can get (we don't have much), and a grain free diet consisting of alfalfa pellets, flax meal, and a vitamin/mineral supplement. I have my own custom vitamin mixed by Uckele Nutrition. It's only $0.41 a day to feed and I have all of the vit/min my horses need in it, without any they don't. I also have them add extra Biotin and Amino Acids, since we don't have much grazing and I feed grass hay. You could call and speak with the product developers at Uckele and see what they recommend for a growing horse. Be sure to tell them what kind of hay he's getting, how much, any pasture/grass, and other feeds or pellets.
Since going grain-free, my horses hold their weight better, have even better foot growth, and their coats just glisten. My "hard keepers" need 1/3rd the food they used to and my moody mares are easier to handle.
Even pelleted feeds use grain products, like wheat middlings, corn gluten, grain by-products, etc., so I stay away from those as well. My hay pellets are hay plus rice bran or bentonite as binding agents, that's it!
Alfalfa pellets are high in quality protein and nutrition too, so it's not just a filler and empty calories like beet pulp.