Based on your statement you have a few choices, but you will probably not like any of the options.
First, DH can stay home and everyone else can go. If he is hurting there may not be too much capability with getting in and out rides, anyway. He wouldn't get full enjoyment under the circumstances.
You can rent a car big enough to handle a
scooter. Then, you can pick and drop off the scooter for a one day rental. You might even be able to rent this locally from your area and then you would not have to deal with this in Orlando. In this case you may have to do a two day rental. You will have to make sure about how late you can return the scooter wherever. Most people will rent a scooter early at MK and then stay for the party. This makes it very unlikely you can rent one of their scooters same day just for the party. You can take a chance here, but it's iffy.
You can rent a wheelchair at the MK park. These should be readily available. This will require someone to push him. I pushed my mother for a few trips and it's doable and I'm not in the best shape. You may be able to trade off with others in your party to help if they are capable of pushing. If you go this route I strongly suggest alternating activities like first do a ride and then a show. The show will provide a rest from the pushing. Then repeat with a ride and then a show again. A wheelchair can be a backup plan to
renting a scooter at the park. You may get lucky and get a scooter and then a wheelchair won't be necessary.
You may want to consider another day at Disney and then you may be able to get a two day scooter rental with drop off and pickup at the resort. I know you only live 90 minutes away, but after a late night at MK this may be a better plan. Then you can enjoy a park the next day, too before going home.
If cost is a real factor then DH staying home or getting a wheelchair will be your best options.