You need to talk to him. You get up from your seat and walk towards the cabin.
You can see him busy with his disgusting hobby.
You retrace your steps immediately. You give him sometime to get over with it.
You come back after 5 minutes. You are thankful that he is writing something.
You get in and speak to him, careful that you do not give him an opportunity to shake hands with you or touch your book/ pen. You are not even looking at him while talking since you are so vigilant about not touching any stray nose boogers rolled away on the desk.