There is cognitive therapy available. It was not covered by our health plan but did help. A neuropsyc exam might be beneficial both to him and to establish his ability to cope with teens. In many ways a stroke seems to be similar to dementia when it comes to personality; it seems to counter a person's ability to compensate for undesirable/unhealthy behaviors and irrational, at least to a fully functioning person, conduct.
Your children need to realize that as their father ages, they are likely to become his caregivers and decision makers. They surely would not leave him to fend for himself homeless and on the streets. You need to position them to be more knowledgeable and sympathetic to his situation.
Their, and by extension, your issues are not only legal but medical. I hope you have a good relationship with his family so you can make your concerns known to them as they are more likely to be in the position to obtain appropriate medical evaluation at this time.
Also, your children are old enough that they are capable of handling the situations that come up unless they have mental health/competency issues themselves. I would hate to think that they would abandon him at the earliest opportunity rather than step up and deal with their dad's situation when they become of legal age. He deserves better and so do they. Help them prepare themselves to be compassionate, responsible adults.