Feeding corn is a very debatable subject, mostly the old-timers feed corn and their horses did and still do fine on it.
I kept my horse at an old farmers' place for the last 4-5 years and it was always a difference of opinion with him. He fed corn and oats mixed with pellets, and extra corn in the winter.
I do not feed corn except for this time of year in Ohio when I pick some out of the field and give it to them as a treat. I buy pelleted feed at the store and they only get about a cup in the morning and evening plus a lot of grass hay, not too rich.
There are lots of things you can teach and do with Jimmy, I saw part of a mini-show once and they had a little trail course set up. A tiny bridge, an overhead arch with streamers hanging down to walk through, back through poles, water crossings, cones to weave. And a lot more. Get a teeny cart, or make a back pack and he can carry your books/groceries, or tote back forage you find for him on walks.
I work at a hotel and had to take a ADA course (Americans with Disabilities Act) where I learned that Miniature horses are approved for service animals, and the hotel is required to allow them in the same as service dogs. There is a vast amount of things you and he can do in your lifetime together.