Misrepresentation of credentials on LinkedIn
If you are sure he is a master student, you could report him on LinkedIn and give him a chance of repenting and stop misrepresenting being a masters student. 

Because, if you report it on LinkedIn (there is a anonymous form for it ,kind of hidden, in their help menu - you have to login but the person you report never knows it was you), after you contest their credential, LinkedIn will ask them for a proof (most of the times), if they don't provide proof they are allowed to just remove and forget the content (yes, they can get caught and go free), but if they do prove it they can keep the information.

Or else, you could email the ombudsman of the department or university and take more formal (and probably harsher) attitude, because it probably goes against the university code of conduct/ethics.