He's not getting enough food.
Since he is unable to eat normal hay, you need to get hay cubes or pellets and soak them for him. There's 30yr old pony where I board that has about 3 teeth, she gets Senior feed, beet pulp and timothy hay pellets all soaked together. If her weight dips at all in Winter, I add Alfalfa pellets to the mix as well.
Morning feed person soaks up enough food that we leave a big ol' bucket out for her and she "grazes" on it all day. I then fill a big feed pan at night for in her stall so she has food overnight.
When it's bitter cold, I also throw a few flakes of alfalfa in her stall, she can't eat the stems but she will pick off and eat all the lil leaves. Keeps her eating (and warm) so she doesn't drop weight.