Does impurity transfer from a contaminated pen to the father's hand and then to other things he touches, and what should someone afflicted with obsessive-compulsive disorder do in this situation?

This act is ugly and reprehensible, so do not repeat it. If the impurity was dry and your father's hand touched the pen while it was dry, then the impurity does not transfer to your father's hand. When in doubt about the transfer of impurity, the original presumption is that it has not transferred. As for obsessive thoughts, their cure is to turn away from them and not pay them any attention.