When were his teeth floated last? If they are not in great shape, he won't be able to chew his food well enough to get any nutrition out of them.
Have you tried a mash instead of grain? I love Purina Equine Senior, good balance of nutrition and it's designed to make a mash -- add warm water and in 5-10 minutes you have a nice mash that an older horse with poor teeth can eat. It is designed to be a complete nutrition for horses that can't eat hay, with feeding guidelines based on weight on the bag. (Def. Check with your vet before making a huge change to his feeding regimen though, and you'll want to gradually build him up to the full amount). Ie - for a 1000 lb horse (on only the feed), they get 13.5 lbs per day. Start out with 2-3 lbs though (I'd go for equivalent molasses to what he is getting), and gradually add another pound a day each week until he's up to the full amount.
If he is able to eat hay, add alfalfa for extra protein. You want to concentrate on protein first, fat second, for healthy weight gain. NOT sugar/molasses.
And even though it may look like he's eating the hay - he may not be digesting it well. If you can find some manure of his that's dried out a bit, pull it apart with a couple sticks. If you see longer pieces of hay in it, then he isn't digesting the food, which means he isn't getting the calories & nutrients he needs from it. Could be that his teeth need attention, or it could be that he's simply to old to chew/digest the hay properly.