He's a bully, either way you put it.... Bullies are cowards, they feel inferior so that's why they try to put someone down, I had a similar problem but with a different issue, I got so fed up with this guys shit that I kicked him in the ass and finally he got the message, never a word again, and, strange as it may sound, now we are the best of buddies.... I'm not advising that you hit or kick him, that worked for me, but like other people say in here, make eye contact and with a firm tone of voice tell him to lay off... I recommend pulling him aside and try talk to him alone....