If we go he will need to be transferred from his wheel chair to a dental exam chair which would trigger the falling backwards dizziness. This situation might prevent examination and cause him to become agitated and upset, or sick.
I recommend calling the dentist's office. They may have a system in place to assist him. They can give you a recommendation based on his age, other issues, how they may assist or take care of him, or even recommend cancelling it.
If he isn't having major dental issues then I would cancel. Just one more stress the both of you don't need. Just brush his teeth twice a day and if he'll spit out and not swallow, use mouthwash. Mom doesn't know to spit so we're using Toms All Natural Kids toothpaste w/o fluoride . No issues if she swallows. Wishing you the best.
If the dentist not willing to do it in a wheelchair, then I wouldn't bother. Especially if u are looking at a 6 month exam and prior visits there have been no problems. If u do go, I would opt out of xrays. The hygienist can find cavities and will bring them to the dentists attn. If small, I would just have them watched. If the visit doesn't go well, depending on age, I wouldn't do it again.