Yes, you need to tell him now. There is another current thread on this site, written by a man whose wife had a brief affair 15 years ago, before they were married, but after they were engaged. Read that thread, and listen to the pain in that man's words as he talks about what his wife did - also understand the misery his wife was bearing for the last 15 years, and how it all came out as the result of a death bed confession... you really don't want to be in that position. Sad, truly sad for all concerned.
see the review