This is totally the kind of letter a doctor's office writes to help a patient get a scooter or a parking space. They aren't going to deny patients those letters unless they have some really good evidence that the patient is faking, and that's hard to prove with claims like intermittent neurological problems. The doctor just signs that shit, usually it is a receptionist or a medical assistant that writes it.
He has a few different health problems for sure, but I really doubt that a rascal is the vehicle of choice when your legs go out on you for 15 minutes at a time. Imagine if your leg went numb and slipped out onto the floor on one of those things while you drove it, you would be fucked. Wheelchairs have more of a bucket type of seat and can come with seat belts to keep people from slipping out. They also have spots to secure legs so they don't fall out or catch on anything, and a rascal does not.