Horses were made to live outside all the time. When domesticated horses came about, the hardy horses still live outside all year round. Some of the fine skinned horses like thoroughbreds started living indoors because people felt it was bad for them to be outside all year. Your horse is perfectly fine living outside. Just make sure he has some form of shelter, whether it be a tree, an outdoor walk in shelter, or a big bush protecting him from the wind. You will have a larger job grooming, as horses don't stay clean outside. Make sure your horse has PLENTY of fresh water to drink. Try to keep AT LEAST 2 buckets out in the field. But you will have to constantly fill it up. I would recommend having a trough of tub of water for him to drink. If you are going to be riding him, make sure you give him a grain supplement to suit his needs. He will need a farrier to check on him regularly, a vet check up regularly, de-worming, vaccinations, ect. Make sure there are NO POISONOUS plants in the field for him to eat, including acorns, ragwort, and other poisonous plants. Check the fence weekly, every other week or as much as possible for broken rails, nails sticking out, or any other hazardous fence parts. Over all, your horse is perfectly capable of staying outside all summer and he would probably be much happier that way!