We use soya meal as a protein boost for encouraging growth height wise, not a weight builder. With young growing horses you may have to deworm more than you would with an adult horse and get feeds formulated specifically for a growing youngster, higher fat, protein, I also use a grow colt supplement along with 24/7 pasture + hay as needed to supplement pasturage. Another thing with youngsters as they shed/get new teeth they may drop weight temporarily as they're doing this, sometimes if a cap doesnt shed properly they cant chew well so that may need to be checked too.