My grandparents build their home some 60 years ago. They have both passed away and the neighbor had the property surveyed and found that part of the house is on his land. He is now claiming that the property belongs to him since it has been on his property so long. I live in Mississippi. Does that sound reasonable to you?

No. Actually it works the other way around. I would be very surprised if Mississippi law doesn't have adverse possession statutes which, in effect, provide that whoever now owns the property (having inherited from your grandparents) own the land free-and-clear of any claim by the neighbor. This is assuming, of course, that your grand-parents thought the land was theirs, and occupied it continiously and exclusively for all those years (not by permission of the neighbor).

How much of the neighbor's land they adversely possessed would be the real issue.

The law in Mississippi regarding ownership by adverse possession is as follows:

SEC. 15-1-13. Ten years' adverse possession gives title.

Ten years' actual adverse possession by any person claiming to be the owner for that time of any land, uninterruptedly continued for ten years by occupancy, descent, conveyance, or otherwise, in whatever way such occupancy may have commenced or continued, shall vest in every actual occupant or possessor of such land a full and complete title, saving to persons under the disability of minority or unsoundness of mind the right to sue within ten years after the removal of such disability, as provided in section 15-1-7. However, the saving in favor of persons under disability of unsoundness of mind shall never extend longer than thirty-one years.

15-1-13. Ten years' adverse possession gives title. (http://www.mscode.com/free/statutes/15/001/0013.htm)

Get an attorney and file an action to quiet title.