I agree with SunnyBlack about contacting your vet about special dietary needs for the foal and taking a look at him in general.
I don't know what type of feed regimen you have him on right now, and this is only a suggestion and opinion, hopefully with your vet's approval: giving the foal free choice grass hay, and if available, small amounts of alfalfa (handfuls for the first few days, and working up gradually but not exceeding more than a couple of lbs. Per day, divided into 2 to 3 feedings, at his age).
Again, with your vet's approval, a foal feed in pelleted form can be soaked into a mash for him, and feedings given 2 to 3 times daily, ideally.
Please be sure to feed by weight, not by the scoop, according to the foal's weight and specifications by your vet. If you can borrow a scale to use to weigh both hay and any supplemental feed properly, that is the only way to ensure you're feeding the appropriate amount based on the animal's weight. Always being sure to weigh the container you use as well so that's taken into consideration. You'll find you won't need the scale after awhile.
Horses will gain on average 1 bcs point a month, and not being in too big a hurry to see improvement is a good idea; it's easy to want to do too much too soon, so patience is ever the watchword :)
Also, a salt and trace mineral block is an essential. I hope something here is of help to you. :)