ID's a pill w/ no markings is extreemly dificult to do, and would probably require a GC/MS test in a lab to determine their nature. It is correct that all perscription and OTC drugs made and distributed in the US are required by law to have identifying markings unique to that pill. This means that the pills your son has are either:
1. a suppliment or vitamin tablet, which are not required to have ID markings
2. a foreign med (foreign meds are not required to be marked, and if it is foreign it could be almost Anything)
3. and Illicit pill (like ecstacy, 2cB,
amphetamines or other substances commonly pressed into pills for the black market)
4. Some orally dissolving tablets (esp.
morphine) are availiable in the US as unmarked white disks. I don't know why these are allowed to be distributed w/o markings, but they are out there.
I know that this range of possiblitys is literally anything from benign to something dangerous and addictive. I don't know where the layman can get lab testing done, and I'm sure if you find a place it will be very expensive.
www.ecstacydata.org also offers a pill testing service for about $40 (i think). while they mostly do ecstacy or suspected ecstacy, they have tested other things including unmarked pills. Give them an e-mail and see if they do testing for other substances, and what substances. I'm sorry I couldn't be of more help, but unmarked pills are THE WORST for folks like us who track down ID's.
Information = Freedom