The way the law views 529 accounts, the beneficiary has no control over access to the funds. If they were of sound mind and chose to deny him the funds, they have that right to do so.
That said, they are not of sound mind and appear unable to adequately manage their affairs.
As there is a medical diagnosis of dementia and their actions show them unable to take care of themselves (leaving the house and getting lost that easily could result in someone get hurt or killed) the best option here is to petition the superior court for a conservatorship over them both.
A conservatorship takes away in its entirety their ability to manage his own affairs and places it in the hands of someone competent to do so. In this situation, you would propose yourself as conservator.
The process is outlined here
That would allow you to decide how the accounts are used, including the 529 they designated for your son's benefit.
If you have more questions please feel free to ask. I'll be happy to answer them. If you do not, please do remember to rate my service in assistance to you today.