Not knowing your state laws. Unless he informed you (x number of years at least). That can become the new property line in some states. I own some land that is @9 ft of neighbors land inside my fence, other side @9’ of my land is on that neighbors side of fence. 5 pieces were done wrong +70 some yrs ago. Long as nobody gets excited it’s ok. One guy got excited. Then he found out HIS LAND is the one that will lose sq footage. Nearly a acre. If it’s “corrected”. For some reason it became a non issue.