Yeah, for a twelve year old it seems pretty normal to get annoyed at being asked about his medication. He probably wants to forget about the fact that he's somehow dependent on taking the medication, so you bringing it up is especially annoying.
My DD started taking Adderall around age 14, so it was a bit easier to get her to tell us how the meds were affecting her. I still find she gets uptight sometimes if I remind her we need to refill her prescription, or if I mention that she shouldn't forget to pack her meds when traveling, etc. One thing that helps is talking to her when she's in a relaxed, good mood. I'll just casually ask about how's it going, do the meds seem to help, can she notice a difference with them, etc. I'm really nonchalant and back off quickly if she doesn't seem receptive, and then I try again at another time. This usually works to get the needed information out of her. Hope this is of some help.