I recently noticed dark spots on my 14 year-old son's tongue. What does that mean? Is that something I should be concerned about?:

You should have it checked by your son's Pediatrician. It is most likely not something to be concerned about, but, you should get it checked out nonetheless. It is probably related to something called "Hairy Tongue".

The tongue has very small stalks made up of keratin, which is the building block for hair. It really isn't hair, but sometimes these stalks can grow taller and attract infection or dirt, etc. Resolution would be cleaning the tongue, perhaps antibiotics.

You are searching on the internet, so you must have already come up with Oral Cancer. This is rare, and you shouldn't be panicked about it. Go see the Doctor, and I'm sure everything will be fine.