Well just for some info, chesnuts are not painful, they have no feeling at all, horse have one on each leg. They are like fingernails, they are dead tissue, no pain. They grow long if they aren't peeled when groomed but even when long it poses no issue for the horse. Horses don't eat fruits and vegetables, they eat hay and/or grass. They ONLY need grain supplments if they are working hard and/or undernourished. If he's overweight, he needs nothing else for a supplement.

If he has trees for shelter he's doing fine. Horses can live even in winter with no shelter or blanket, although it would be best for him to have a lean to to go under to get out of the wind, they do not need to be stabled at all. My horses live outside 24/7. They have a lean to but they are healthier (respiratory wise) and happier living outside, rather than in stalls.

The horse should be fed twice a day, morning and night, if he's only eating hay. If there is pasture grass he probably doesn't need the hay in the summer/fall, only winter. He needs a clean water supply daily, a large trough filled with water that is kept full and dumped and cleaned out at least once a week.

I would not recommend riding him or doing anything else with him without an experienced horse person to help you, you will probably get hurt otherwise. You can pet him and get him used to you but I would refrain from doing much else with him until you get someone to help you.

Apples are fine, the seeds are fine, but honeslty he doesn't need much else. If you want to feed him something give him a carrot or two as well to "make friends" but he doesn't need any supplemental feeding if hes already overweight.

He would probably just like company and maybe some brushing if he's not scared of it but again, since he has not been handled much, get someone who knows about horses to come help teach you what to do so you are safe. It's really easy to get hurt. Good luck!