I know a pony that is 44 this year. He has no teeth and he does well on super soaked Amplify along with his regular senior feed. Amplify helps hard keepers keep weight on while also helping their hooves and coat. I would definitely try it on this guy.
Increased fat intake is a better choice for these old guys to add extra calories than alfalfa. It is much easier to digest and provides twice the digestible energy as protein. Vegetable oil is an easy way to add fat to the diet. 1-2 cups a day adds considerablely to the "calorie" content of the diet.
Corn oil has shown to cause increased inflammation in arthritic horses and it can decrease nutrient absorption in the gut.
A better way to add fat would be to use stabilized rice bran or milled flax. Both are high in fat content and are more easily digested. Quite a few feeds are now using rice bran to increase the fat content, instead of soybean or corn oil.