Sounds like a pretty typical and normal 13 year old boy....hehe. The idea that 'mom TALKS about 'GROSS THINGS' like sex and body parts IS DISCUSTING and EMBARRASHING. Obviously in the future, if the need araises to 'mention the unmentionable' body parts, you need to lower your head, turn your knees inward, assume a cowardly and embarrashed bemeanor, and say: "you know what I mean.. DOWN THERE! (an appropriate RED FACED BLUSH would also help).
The good news is that in a few years,(if not already) he won't be able to get his mind off of "down there".. hehe. I assume he locks the bathroom door? hehe. Surely you do NOT enter his room without 'giving him time to 'cover up'... and his 'unlocking his door? YOu don't look at his underware when you wash clothes I'm sure.. hehe.