without hurting his feelings

I was at lunch one day with one of my friends.I looked over and saw this guy who i found out was mentally retarted walking to sit down with us.My friend started laughingbut i told her not to judge cause gods gonna end up punishing her one way or another.5 min past and she started laughing again and wispered to me that the guy was starring at my chest and I looked over and saw him looking.When he looked up he started smiling and looked back down again.When i went to go dump my tray he scooted over to my seat and started touching it when he saw me coming bac he scooted back to his seat.I could see in the corner of my eye that he was still looking.When it was finally time to go one of my other friends came up and said that he did the same thing to her and started following her around.I dont know what to do or say to him without me looking like im only saying it because he's mental.I dont know what to doo!!Advice please