How did you determine that your driveway is on his land?
You would probably be best served by seeking a free consult with an attorney. S/he could give you a run-down on applicable laws and statutes, and perhaps estimate what the cost of seeking adverse possession might be.
How big is your lot? Is there room to move that 100 sq ft onto your own property? Has the neighbor tried to stop you using the driveway? Have you checked for an in place easement for the portion on his land?

