When my horse was 5/6 I had a hell of a time keeping weight on him too. When he turned 7 it started to top off - now I have to make sure he is not getting too many calories so watch that too!
Anyways, what worked for us is he has 4 feedings of hay/day at the barn. Each of these we fed about "2 flakes" or 3-5kg of a good quality 30% alfalfa hay. He was on a round bale as well during turnout (grass hay), or he was on grass. Twice a day when they are grained he got 1.5 lbs of a soybean hull based mineral ration and "2 scoops" (about 2lbs dry) of beet pulp.
The mineral ration is very important. He got everything he needed, balanced, plus it is high fat (soybean hull base) and it contains Diamond V yeast culture which aids in digestion and balancing the hind gut. The beet pulp was another filler/weight builder.
Now he is on the same amount of Alfalfa hay, dry turnout, and 1lb ration with about 1/2 cup of rolled oats and a biotin supplement. If he starts dropping weight over winter, I'll add him back onto beet pulp. But right now he is a bit plump lol. So it should end! As soon as they stop growing - then they can easily get fat (usually) so monitor your horse and do a "body score" once in a while.