He will tell you his experiences if you do not register shock or concern but answer each question matter of factly. This is an experiential journey for him and will unfold over years, so answer each question succinctly, do not answer more than is necessary, and ask him in a way you you can make sure he understands your answer at the moment. (Okay, son?) He will naturally want to ask questions of other people, like a friend or a man he feels he can trust to tell him the truth. This may or may not be his dad. It's natural.
My mom (55 years ago) and my older sister made a pretty big issue of my wetting the bed when I was seven (Mom because I tried to hide it and sis because she wanted to make fun of me), and I became very embarrassed and clammed up for a few more years of what became wet dreams (ejaculations). It did not scar me, damage my sex life or make me overly modest. I turned out normal because I had and uncle and aunt who, because they knew my older inrtoverted parents so well, took it upon themselves to have careful, frank discussions with me and answer my questions, and because the doctor who delivered and cared for me until I was 20 was a really cool guy and taught me to take care of myself. I had a really good sex education.
As a man, my best advice to you is this: answer your son honestly and warn him away from jocks and fraternity boys who know little about protecting themselves with the truth and even less about respecting women. He will grow up to be respectful of others mostly because he learns to be respectful of himself. This is how he will learn how and why to apply the Golden Rule. You and your husband are his role models.
Having read six other responses to your question, I would say there is a lot of good advice here. It certainly sounds like your son is quite fortunate to have you as his mom.
Best wishes to you and your family as you help your son on his journey through life.