You can go online and search for pill identifier. Once you get to the site you will have to enter any inscriptions that are on the pill, enter the color and size of the pill. It will come back and tell what the pill is.
Ask your son! But, don't be surprised if he tries to accuse you of snooping, etc., But, if you don't get an answer from him that you're satisfied with, then go to a pharmacy, or check it out on line.
Doe have a change of friends, grades declining, secretive about where he is going and where he is at. Usually if you suspect it as a parent than it is usually true. Let him know it will not be tolerated. Bring his bottom to him by letting him know he will loose you and what he is used to.