How to share my music with my kid's iTunes?
In terms of technology, the files you're talking about don't have DRM, so you can simply copy them to your son's computer. You don't say what kind of devices either of you are using, but you can right-click songs in iTunes and choose Show in Finder/Windows Explorer to see where your music files are located. Then you can transfer these files to your son using the same methods as any other files.

But whether it is legal to do that depends on where you live and other factors, and in some cases it may be a grey area. I just want to be clear that I can give you technical advice, but I can't give you legal advice. I am not suggesting that you break any the laws where you live.