In my experience most narcotics carry the makers mark, if you are concerned about what your son is up to.................just ask him, CALMLY. yes he may have disappointed you if they are illegal drugs, but he's still your son and you run the risk of pushing him further away by not being honest. Sorry to say you are up the creek without a paddle if you were snooping in his belongings. Hes a teenager, did you ever do anything like this when you were young, smoke a bit of dope, drink a little too much, he is finding his feet in the world, and he can not do this with mummy holding his hand.
But like all mothers you can not help but interfere, that's what we love about our mums, the fact that no matter how much we stuff up they will always interfere, talk to him, treat him like an adult, explain to him that you DO understand that it may only be a faze that he is going through, trying a little extermination and you understand that. But part of being a grown up in to own up when you stuff up. Explain to him the consequences of drugs, not only the health effects but also the social effects it has not only on him, but on the family and the community as a whole.
Educate yourself, educate him, find out together, tell him you may have lost touch with whats going on and want to find out more, and you need his help in doing so. Sit in front of the computer and take the journey with him, this may make him open up. If you really are concerned with the pills you found though, be weary with the police, you are maybe taking illegal narcotics into a police station which is never a good idea. Depending on the age of your son, and where you live, here in Oz we have the J.A.B juvenile aid bureau, which deal exclusively with young people, hence their name, ask to speak to an officer, and tell them your concerns, and what assistance they can offer to you, they may be aware of a drug program which teaches and educates young people about the effects of drugs better then you can.
I will reiterate the most important thing, be CALM, if he is into drugs for reasons other then extermination, you may push him away further if you rant and rave, so be honest with him, if you tell him about your childhood and growing up and the stupid things you did, try to make him understand that you are not angry with him, (yes you are but he don't have to know) that your concerned with him, and no matter what he tells you, you will not judge him, punish him (unless its really f*&^ed up) but you want to help him, to listen to him, to hear his side of the story before you both decide how to move on and take the next step.
A site you may find helpful in identifying the substance
Former causal drug user on the wrong track set straight by a loving an supportive mother, Love ya mum