There is no way to skip any lines in WDW. There is a GAC which can help with invisible issues, such as a sun allergy, but there is no pass to shorten wait times. Also the GAC does not work for buses, character lines, restaurants, anything at the resorts, stores, parades, fireworks, shows, etc.
It sounds as if he will require a wheelchair or
ECV (driving an ECV can be very tiring). To give you an idea, walking ONCE around the World Showcase lagoon is 1.25 miles (not including walking into any shows or pavilions). Add another mile to walk in and out of the park either from parking or from the bus, and it can be downright dangerous. (I have a condition that causes spontaneous aortic aneurysms)
Also, many of the shows and street entertainment means that he will have to stand for that.
If you are taking the bus, the only way to ensure that he will be able to sit on the bus is if he has a wheelchair.
There is probably no way he can do this without having a wheelchair. I would look into renting one off site. There is a list of companies on the FAQs on the top of the forum.