I am guessing that you claim you have adverse possession and he claims you don;t but a court has not resolved the dispute yet.
I am also guessing that he has contacted an attorney and has been advised to be forceful in this situation to protect his rights and diminish your adverse possession claim.
You can try law enforcement but they will say it is a civil suit most likely and not get involved. Maybe not if you say you are contacting an attorney on Monday and you just want to keep the peace until the attorney can get to court.
Two approaches come to mind. One is to tell him to stay off your land with the same force that he told you to get off his. Maybe he is bluffing and has no stomach for further action. My preferred method though would be to tell him you will contact an attorney Monday and that the attorney and neighbor can work out the issues. You will probably find out who his attorney is in the process.
Try to keep a lid on the upset because he is your neighbor and depending on the situation, you may need his help someday and he yours. Boundary disputes with warring neighbors can truly diminish the quality of each owners life all for a piece of land that may not be as valuable as you think. Definitely pursue your rights if you really think it is yours by adverse possession but don't make it a cause celebre.
Adverse possession claims are very fact specific so it is not possible to say which one of you has a better claim. You will need to consult with a real estate attorney who is very well versed in that area. There are a number on your side of the lake. Hopefully they will respond to you here so that you have some names to call.