Help me and my beautiful pony?

I wouldn't leave him in a stall longer, because he will lose muscle and may start bad habits out of boredom, like cribbing. Taking long rides and pushing him enough to sweat will help him take some weight off. Lunging about a forty-five minutes 3-4 times a week will help build muscle, replacing the fat. Checking what types of grass grow in the pasture might be helpful because some grasses are only healthy in small amounts and in large amounts might make them gain weight. I know someone said this already, but using a grazing muzzle will help decrease the grass they eat in the pasture, especially, as I said before, it is too rich of a grass. If none of this helps, you might want to consult your vet and ask about what might help him lose the weight and put him on a good nutrition plan. Watch him to make sure he is getting enough exercise in the pasture and has a large area to run in.