Agree that "before" pictures - along with progress pictures - would be quite helpful in being able to evaluate his progress. Working closely with your vet is key in a rehab - what has your's had to say thus far about his progress, etc?
While I realize you said he has grass/pasture - my preference when putting weight on a horse is to have free choice hay available, even in a pasture situation. "2 flakes" really isn't very descriptive - what type of hay (grass - again, what type - or alfalfa?), how much does a flake weigh, etc. To maintain weight the common accepted formula of roughage intake is 1-2% of the horse's bodyweight - to put weight ON you need more, which is why knowing how much the hay you are giving weighs is helpful.
Are you planning to geld him? Doing so sooner rather than later will be beneficial as you can change the track of his thinking.