First,
Stop asking for legal advice from random strangers on the internet.
If you do something based on such advice, you may be the one ending up in front of a judge facing criminal charges of your own, and what is your defense going to be? "Some guys on the internet said grab a shotgun" or do ABC? How much of that do you think will fly in the eyes of the legal system?
Every state has its own laws, and within counties and jurisdictions you may have different procedures.
Call the county or city in which you reside and ask to speak with a police officer or civilian service officer, explain your situation and go from there. If you don't get resolution, go to the county and ask to speak in person with a victim witness coordinator ,etc.
Are you physically afraid of your grandson as he is much younger and stronger, and you have concerns for your safety and your wife's safety? Are you physically being intimidated by this person? Has he taken money and not paid it back? Has he stolen items? Exploitation of the elderly is frowned upon very heavily by the courts.
Don't pull the tough guy typical campfire BS.
You will get further going in a different direction. But don't take legal advice from strangers. Go get actual advice and services from agencies in the jurisdiction where you live. The courts generally tend to protect the elderly and have numerous agencies and advocates within agencies that deal directly with this sort of thing.
Use them to your advantage.
Unfortunately, your wife’s grandson may have tenancy rights, as was mentioned by at least one poster above, when they said an eviction may be required. So, don’t just pack up their belongings and put it in a storage unit with just a month paid for. Do as McS says, and talk to local LE about your options, but be prepared to have to go through the eviction process. Assuming that the GS manages to play nice. If he doesn’t, well then all bets are off.
