My horse dropped a lot of weight and I switched boarders because of it. Luckily, I moved him before he was too bad, and he actually looked about like your boy (I'll post pictures soon). It's taken me months to get him to where I'd like him, and now he's even a little chubby because I've given him the last 2 months off for an unrelated issue.
From my experience, I'd say your boy is at a "healthy" weight - I still rode him in 25-mile endurance rides at this weight with the "ok" from both of the vets at the ride. However, he'll finally be "filled out" when those hips don't stick out so far. I think my boy is a little bony in the hip, but he finally filled in those gaps so it didn't look like he had giant points sticking out. What finally did the trick and put those last few pounds on were 24/7 free-feeding on grass hay and a supplement mix that included rice bran, wheat midlings, beet pulp, and a few other things. I didn't feed a ton of grain because he's Arab and I didn't want him to get hot. Certain other mixes are good, too, and I know a lot of riders I know use them to put the weight on. I could ask about those...
Here's what my boy looked like a week after I took him from the old boarder's place. He looked a little worse than this when we left (middle of May):
Here's today's picture (and I keep him in a blanket, so he's not very hairy):
I don't know what he looked like before, but he still needs a couple hundred pounds put on. I hope your not riding him?? He is still very much under weight.
Glad that he has someone like you to help him.
I would up his food intake ( hay ) and add some soaked beet pulp if you can and make sure he is on a regular deworming schedule. If he is over loaded with worms it will be harder to get his weight on.
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.