I would def change from your feed to a ration balancer. A ration balancer will provide all the nutrition they need in just 1lb a day. RB's are awesome for any horse, but def for easy keepers, and they are cheaper.
As far as his weight goes, it is normal for a horse that young to be a tad ribby. He looks fine to me in the pics, but pics can be deceiving. Only you can tell if he really is too thin. How easily can you feel ribs? He is def lacking in muscle on his topline to me and this can be a cause of not getting adequate nutrition and amino acids.
Hay? He should be getting a MIN of 2% of his body weight in hay so unless you get an accurate weight on him, I have no clue how much that would be. You saying that you feed X number of flakes does me no good without knowing how much each one of your flakes weigh.
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