They would store it at First Aid, but in most cases, you want to keep the insulin with you. Mostly because it is inconvenient to go all the way to First Aid to get it. Once the insulin has been given, the person will need to eat fairly quickly or their blood sugar will drop too much.
Many diabetics use something called a
Frio pac to keep their insulin cool while they are out. It cools with water, so there is no need for refrigeration.
You will not be able to rent a power wheelchair - in general, the companies that rent them will only rent them for use by people who already are experienced with using them. You would either be able to rent a manualm (push) wheelchair or an
ECV, also called a power mobility
scooter.
ECVs are much easier to drive than a power wheelchair and would give him some independence. Many stores (Target,
Walmart, grocery stores) have ECVs for shoppers to use. Trying one of them would give him some experience and help him to decide whether using one is something he wants to do.
Depending on what deficits his stroke left him with, he may find that it takes too much energy or concentration to drive an ECV in the parks (you have to watch out for traffic around you, things like curbs and people who dash out in front of you).
You can find out more information about ECVs and wheelchairs (including links and phone numbers for places that rent them) in post #2 of the disABILITIES FAQs thread near the top of the disABILITIES Board. (There is a link to that thread in my signature). In post #3 of that thread, there are links to past helpful threads - you will find some about ECVs in the parks and also about managing diabetes.
Also just look around on the first few pages of the disABILITIES Board; diabetes is a frequent subject.