my 5 year old and the birds and the bees!

Ellie • Engaged to my soulmate💏and newly a SAHM to a 8 year old boy 👶🏽 4 year old girl and my newest baby girl born 1/25/18👶🏽👶🏽🎀🎀 💕
Okay y'all so my 5 almost 6 year old knows about his soon to be baby sibling and asked the big question.. 'how do you make babies!' 
I was completely baffled and caught off guard and daddy did a quick detour to another subject and that was that. 
I am wondering if/ when this comes up again how I say it to my son without all the grown up details..