I had a lab TA interested in me when I was an undergrad (and dinosaurs roamed the earth). Organic chem for the curious. I was seeing someone at the time, so I feigned ignorance. What you do is tell him casually and privately he seems like a great guy and would he be interested in getting together for coffee *after* your course is finished. It would be inappropriate for him to date you while you are a student, but afterward is perfectly fine.
Good luck.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh