Bernard is a trans boy; at school he can wear the clothes he wants and aspire to be the kind of boy he wants to be in high school. His parents don't really care what he wears or how he behaves at school... unless there are parent-teacher events or social parties where they have to participate.
During one of those events, he could no longer bear his parents' treatment and hid in one of the classrooms. Tim found him some time later, and to comfort him, said not to worry, that the three of them (Tim, Darla and Bernard) could go to the prom however they wanted. Tim said he would get him a custom-made tuxedo.
But that never happened; the constant rejection of his family towards him and the distance from his friends led him to end up in the cult in search of comfort.
Time passes, he manages to leave that cult, but he has some aftereffects, so he has very blurry memories of his last year of high school and the time inside the cult. Anyway, Bernard doesn't understand why Tim is so obsessed with buying him custom-made suits for every Wayne Enterprises event or social party he has to attend.