Impossible. If he isn’t wearing a white shirt, he is not yeshivish. It doesn’t matter what he learns, how he behaves, who his friends are, how he respects people. So long as he can schmeff, and wears black and white, he is yeshivish.
Yeshivish is the “lvush.” A person’s pnimious would determine how frum they are. The two dont always go together. The yeshivish person can go around wearing black and white and be yeshivish even though when he goes on vacation he doesnt act so frum yet he still looks yeshivish…Frum is the person’s inside, how the person acts in things bein adam lamakom middos,davening,yiras shamayim etc. he may not look so yeshivish but can be a way better person.
Yeshivish is whatever you are if you want to be Yeshivish, and whatever you aren’t if you don’t want to be Yeshivish.(In other words it’s an arbitrary, subjective gray area, that is defined differently by nearly everyone.)
As per the saintly Rav Meir Shapiro ZTKLLH’H:- Every tractate without exception is a Yeshivish Messechto, hence the need for his inspirational & ingenious Daf Yomi brainwave, so no Messachto gets mislabeled as being more Yeshivish than another Messechto.