I absolutely hate saying things like this, but as a drug addict I know all too well about engaging in those behaviors. There's a good chance he lied to you about using. 'I didn't use any, just got it for a friend.' Know how many times I said that? It was a lie, every time. Lying and addiction go hand in hand. They thrive on each other.
I don't want to be negative. We are all here to help you. The most important thing you can do is take care of yourself, and your baby. Living in constant suspicion and fear, and going through his messages looking for 'evidence' is just no way to live. You deserve more, much more.
Here is a link to the family and friends message board. You will find an overwhelming amount of support there.
Friends and Family of Substance Abusers - SoberRecovery : Alcoholism Drug Addiction Help and Information
Please stay, post, ask questions, and read. I truly wish you the best, because you deserve to be happy.
EDIT: I see you already found the F&F boards. You are in the right place.