Body language could do the trick. When he starts patting, then stiffen up or wiggle your shoulder as if you are pulling away from his hand. Make an uncomfortable face and wince a little. If that doesn't work, then reach up and grab his hand and squeeze his fingers a little so it is slightly uncomfortable. Act as if you are doing that without even thinking. If that doesn't work, then reach over and pat his shoulder as he is patting yours. Ideally, he would be the kind of person that you could just give a few hints to, and he would take the hint. If not, maybe it's best to just say, "Why do you pat my shoulder? It's unusual." Look puzzled or grin.
I don't think so, but you are going to have to send some clear signs that it isn't comfortable for you. Even if it involves stepping away from the hand.
yes as it may be seen as sexual harassment
There shouldn't be touching....when a woman touches a man ever so lightly it has meaning...same goes for a man...he's testing the waters...
Tell him it makes you horny, to please stop.
Put a mousetrap on your shoulder