Lush pasture is a danger zone for ponies. Ponies can founder in less than 60 minutes of grazing if introduced suddenly to lush grass. If you plan to keep your pony on grass introduce it very slowly. Start with 10 minutes of grazing and gradually add a few minutes each day twice a day. If your pony eats too much rich pasture it could lead to such things as colic or founder. You may never be able to leave some ponies on good pasture. Ponies can become obese very quickly and that can lead to health problems.
Good pasture for a pony would be one that he has to work at finding the grass in. Sparse grass that grows slowly would be ideal. Or he could spend a small portion of his time on pasture and the rest in a grassless paddock. Some people use their round pen or a paddock where no grass grows. Another option is to use a grazing muzzle. Make sure your pony has access to clean fresh water.
Ponies rarely need concentrates or grain. The exception would be a pony that is working hard: one that is doing several lessons a week, is being driven frequently, is doing something like pulling competitions, or a lactating mare. If your pony is losing condition you could increase the quantity of hay and if that isn't enough add a concentrate that isn't too rich. A forage replacer fortified with vitamins and minerals might give your pony the nutritional boost it needs. Pony mouths are small, so overgrown teeth can be a problem. Don't forget to have your vet check your pony's teeth to ensure it can chew easily. Ponies also need regular de-worming to keep them in the best health.
If you like to feed your ponies even if it is not working hard--and for some owners this is a very satisfying activity--look for a concentrate that is low in calories. Some manufacturers make special pony mixes. These mixes are balanced with the correct amount of supplements for a pony. Don't be tempted to top dress it with a lot of extras like molasses or beet pulp. If you are feeding good hay, the pony is getting a bit of pasture and you have a mineral/salt block available your pony will be getting what he needs.