In order to keep your base strong, first approach your GF, ask her if she have ANY feelings for this guy too. Convince her to tell the truth, even if you might not want to dare hear the unfavourable answer. Should she reply with a NO, just walkup straight to that guy, & unveil your relationship status. Tell him everything & that will be his first change. Next time he again does the same, ask your GF to go up directly to higher authorities & complain about this guy of giving her wierd looks.

~noms~
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"Before God we are all equally wise ' and equally foolish" - Albert Einstein