I agree, you should have his teeth checked. To put weight on I would make sure he has plenty of good quality hay infront of him most of the time, sweet feed is ok but it can give them more energy and cause them to burn more calories. You can continue to feed the sweet feed but I would add oats to it half/half. I would also give him 10 cc's of probios paste twice a week for a month and then once a week to maintan him, the probios will ballance the stomach and allow him to uptake more nutiance from his feed.
To add muscle you must up his protien.
hope this helps.
Cathy C