Hiya, Jen!
Walt Disney World is a lot, so first, look into Disney's
Disability Access Service or DAS. This is meant to make it easier for guests with disabilities to wait for an attraction. You can
pre-register for DAS to make sure you qualify and to begin planning for your upcoming visit. You have to do this at least two days prior to arriving, so now's the time to check it out. They'll ask what type of accommodations your brother-in-law may need DAS to help with, so give that some thought ahead of time.
Definitely consider
renting an Electric Conveyance Vehicle (scooter). You are visiting at a busy time with peak heat and humidity, and a scooter is going to take away the stress of walking and standing, especially if your brother-in-law tires easily. They are available to rent at any theme park, and you can even take your receipt if you plan on
park hopping so that you can rent at your new park at no additional cost.
Plan lots of downtime. Visit the parks early in the day and give him a chance to go back to your
resort and rest or relax by the pool. Make indoor
dining plans so there's a place to cool off at least part of the day, and if meeting characters is important, try to plan character meals to minimize time waiting in lines out in the heat.
Jen, make all the memories, have all the fun, and enjoy all the time you have together as a family. These moments will become magical, cherished memories, and I'm so glad you'll be able to experience them together.
Anna