There are two things involved here ~ what your boyfriend's fraternity does, and what your sorority does.
I don't know what Sig Ep does ~ as I've never dated a Sig Ep ~ nor have I ever been lavaliered by an active member of any fraternity. What the local Sig Ep chapter does (if applicable) is really up to them. Your boyfriend should inquire with the local collegiate chapter, or if none, with the local alum chapter. If they don't want to do anything, I would suggest your bf just buy the lavalier and present it to you.
As for your sorority ~ I can only share my story. My husband is an independent. He learned about the lavaliering ritual, but since he is not a member of a fraternity, he couldn't "lavalier" me in the traditional sense. Instead, he chose a pendant, a little gold heart, and gave that to me as a "lavalier". My sorority then held a formal candle-lighting ceremony for me, just as if I had been lavaliered by a fraternity member. (BTW, I was my chapter's ritualist at the time ~ so saying "guess what, we're lavaliering someone tonight" held a lot of weight
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