That is a difficult situation indeed. As soon as possible get the ball rolling with a wrongful detainer so he knows you are serious. Things are pretty backlogged so who knows when they will get to it.
You could also try calling the police and see if that will shake this person up a bit. Realistically, the police will tell you that you need to file a wrongful detainer. It would essentially be posturing on your part which could end up back firing because it will reinforce his position that you really can't get him out. You can turn off the power to make his life a little tougher and without a lease he shouldn't be able to get it back on.
It is all a delicate balance because he is inside your property and can make your life difficult by drawing this out and potentially doing physical damage.
If I were in your situation I would begin having a discussion of cash for keys. Start low at $500 and you will most likely need to land between $2500 and $5000 which is the amount of money that sophisticated squatters expect given the challenges around eviction at the moment.
So no good options and potentially an expensive lesson...I haven't been in this particular situation before but I have had expensive lessons in Baltimore and my main in advice is remove emotion and look at this as objectively as possible. This person is clearly a piece of **** but just look at it as the cost of doing business, recognize you made a mistake, sell this property, and move on.