C.D.
Hi L.,
I can offer you the solution that my mom used on me. You will have to stand by to make sure that his finger does not rub his eyes though. Simply put some habenero, or hot pepper sauce on the finger. This should work within a couple of tries. Also, when he puts his hot sauce finger in his mouth tell him "No more thumb in mouth"!
Best wishes,
C.
Mom of three, grandma of 2