First, with a young horse you want a steady increase of body weight and height - no rushed growth spurs that can later cause problems. No rapid changes in feed, and as said above, try to purchase the feed made locally.
– Good quality protein feeds
– Low starch
– Good vitamin and mineral levels
– Good quality forage (e.g. Grass)
– Feed a young-stock pellet (if required)
– Good doers = balancer only
Basically, give good quality hay, have a salt/mineral block on the side, ad lib forage, and feed forage balancer - can add in some vitamins and minerals, if sure you need them. Can on the side feed some sugar beet pulp for more protein, some alfalfa, but they do not really need any grain - that will just give too much energy for growth spurs.
Most youngsters don't need much more than forage (hay/grass) and some balancing as they aren't in work, they should grow slowly and develop at their own rate, later when starting to go in work, can add more energy dense feeds - but doesn't have to be grain, and work your way from there :)