Dear @Mfj02,
Welcome back to the Wefugees community and thank you for your question.
Unfortunately, I can not share experiences regarding similar cases (except a question on the middlename which my colleague was answering here), but I would agree with @Emmytender: If the immigration office asks about the middlename, explain the situation exactly how you explained it to us. I suppose that your boyfriend also has a birth certificate since he just applied for a new passport? Is the birth certificate showing the middle name? This could also help in case the immigration office has any doubts. In general, I would not worry too much.
Please don't hesitate to keep us updated on your case or to get back to us with any further questions.
All the best,
Meike