Yes, sounds like he had the talk at home. You can always ask the parents if that's the case. Sometimes rather they've picked it up from an older cousin or something - so they might appreciate knowing what he's saying.
This will happen to you over and over again as time goes on - they will come home from school hearing stuff - you can't stop it.
My son once came home from preschool very upset he didn't have a vagina.
How did your kids respond? Usually mine don't even take notice at that age - it sort of goes over their heads. Kids will throw things out they've heard at home but they aren't always even sure what it all means.
If my kids don't respond, I let it go because me getting involved actually draws more attention to it. If it's a bit awkward, I just change the subject as casually as possible.
My boys were used to me breastfeeding. When they heard about "boobs" at school, they didn't associate it with breastfeeding - they seemed to know enough that one didn't mean the other (that one could be private and one was for food).
Zoos are actually pretty good places for talking about birds and bees because it's often right there in front of you. Still remember my little guys checking out these squirrels "playing together" while the rest of us were watching monkeys. Just so long as they lead with the questions and it's age appropriate. I only ever give as much info as they can handle and I always let them ask. Later on, we got into more detail.
If it became a problem with the child always talking sex - then I would deal with it. Likely he just heard the talk recently and will move on to something else next time to talk about.
Hope that helps!