Why would my son be taking pens and mechanical pencils apart?

I can think of a whole host of reasons for the use of the pens and pencils. If I were you, I'd come right out and ask him. Then, to verify his reply (I hope you're not one of those parents that think their children never lie to them), take him to a physician with the devil's advocate in hand and ask to have the contents remaining in the dispensers analyzed by a laboratory.

Your son is using them to snort pills. The fact that he doesn't even try to cover his habit up is disturbing. If you don't support his actions, you need to take your home back!! There is residue from whatever pill he's snorted in that pen. I am sure you know deep down what you need to do. It's HIS habit don't take responsiblity for it! He needs help honey, but you need to survive too... Good Luck Sweetpea

The only thing I can think of is if he is using the pen barrell to snort crushed pills with.