He still needs weight. Have you had him gelded? That can really help with weight gain.
As to how you are feeding.. "two flakes" of hay is not meaningful. Get a scale and weigh the hay. Also.. you need to have that hay tested to determine what is in it. Pretty looking 1st cutting hay can be low protien, low energy, high neutral detergent fiber stuff that mostly passes through the horse with little sticking on the way.
Hay value is determined by the stage of maturity of the grass or legume when it is cut, the quality of curing (dusty or mouldy hay can not ony make the horse sick but ties up the nutrients so the animal cannot use them) and the type of grass or legume (or mix) in the hay. It also matters if the hay is first cutting or second cutting and so forth.
In the northeast US first cutting hay should be cut before June 1 for most legumes and grasses with the exception of Orchard grass and Reed Canary grass which must be cut before May 25 to have feed value. For every day most hay is cut after June 1 you lose around 1/2% protien per day and the NDF increases. Once grass hay heads out and blooms (or legumes bloom) the feed quality goes down considerably along with digestibility. Of course this also is dependent on Growing degree days and they like.. but June 1 is the general cut off.
Weigh your hay and test your hay and balance the ration based on the nutritional value... and forget "flakes." Flakes are determined by the ground speed of the baler, the size of the windrow and the RPM of the baler which determines the plunger speed. Baler is designed to run at 540 RPM (I ran mine a little slower). Second cutting "flakes" (smaller windrows, shorter stemmed grasses and so forth) were VASTLY different weight and size than First cutting "flakes." Grass hay flakes were vastly different than Alfalfa and mixes in between were different as well.
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer