i really need advice on how to approach this conversation

I'm sorry if this is long but someone please offer advice. Posting anonymously just because it's embarrassing and I'm to my breaking point. I'm 38 weeks pregnant, and I've been with my boyfriend for a year and a half. We moved into our own house 3 months ago and he got a good job and I stay at home. Before this, he went to school almost 2 hours away from me. While he was there he was big into taking pills and it almost tore us apart but I stuck around. I was so happy when we finally moved into our house and got him out of that terrible town, but just my luck, guys at his new job are into these same things. He goes to work everyday and provides for me and I really truly haven't seen him high for a long time (trust me I know, I was around it for long enough) but I don't understand why almost daily I'm finding texts in his phone to his work buddies asking if they "have anything" etc... I've caught him so many times in the past 3 months but he always has some great story and of course I believe it. But I'm just so sick of feeling lied to and betrayed. I've never gone behind his back, I literally stay home all day and can't go anywhere because he uses my car. His background... he's a marine veteran and he has seen combat so I feel deep down he has issues that he's embarrassed to talk about and he needs the pills to cope with his own mind. Which I understand but he won't admit that to me. We've fought about this countless times and everytime he just gets pissed and it gets no where. But I'm to the point that I need to get through to him. I am MORE than willing to help him through this but I can never seem to get him to understand this. I can never find the right way to have this conversation with him, but I need to now more than ever because our son is about to be born and I want to know where we stand and I especially don't want our son around this. Please help me with how to go about this. Oh and as an update, I found him once pretty much dead.. he wa purple and not breathing and I can't handle that and I invision it in my head constantly.