Your 9-10 year-old son is likely becoming curious about his boy body. He has entered puberty and has begun to notice changes.
During these years, it is good to teach him one small topic at a time as he matures through puberty so he receives accurate information about what is going on. One topic you can share with him is the change going on inside his testicles. Until puberty starts, not much is happening there. But now, sperm are beginning to be produced in tiny little tubes inside the testicles.
In this anatomy figure from my book, you can see stacks of small round cells in clumps in the wall of this tube. There is a white space on the middle of the tube. The round cells under the yellow layer keep changing and changing as they are pushed toward the white space in the center. By the time they reach the white space, the cells look completely different than when they started. Now, they look like sperm.
There are several hundred of these tubes in each testicle all making sperm at the same time. It is probably not surprising to learn that when your son is sexually mature, in a few years, his testicles can produce about 1,000 sperm a second!