I believe there is also a sticky called 'what addicts do' at the top. This was a real eye opener for me, take a look at it.
Your boyfriend sounds eerily similiar to my RAH when he was still using. He is also an alcoholic but his drug of choice was always pot. Towards the end though, he started taking different pills here and there. I, like you, thought that this wasn't his preference anyway and when I found one in his pocket and he told me 'someone gave it to him and its the only one he has', I believed him. What I didn't really realize until later is that addicts will typically use whatever they can to get high. I've seen some folks on here even say they resorted to cooking wine or mouthwash! It took me a long time to understand this and to also realize the truth so I understand where you are coming from. Also, he will likely NOT ask you about the pills you took but instead just act like nothing has happened....