I would suggest grass hay 24/7, alfalfa two or three times a day, and the feed at least two times a day; three or four would be better.
You can add rice bran, beet pulp (soaked), alfalfa pellets (dry if he doesn't bolt his feed, otherwise soaked), flax seed (good source of omega 3s), or any of a number of commercial weight gain supplements.
I would suggest taking a picture once a week (same day and time if at all possible) so you can keep track of his progress.
When adding things to his diet, do it slowly and one at a time. That way, if he has problems with anything, you know what it is.
I'm not familiar with Blue Seal, but I would check out the vitamin and mineral content of that and the hay and supplement whatever he needs. Make sure he has access to salt and water at all times as well.
Good luck with him, and bless you for taking him on.
Learning never stops