please no rude answers. looking for advice.

I just moved back closer to home to be closer to my family now that I'm pregnant with my 3rd. My first child has a different father (from around here). He was a heroin addict which is the reason I left in the first place. His parents told me he was clean and had a job and was doing very well before I moved back. I let him go to their house for a week (he is supposed to be there for most of the summer). When he came to see my mom, he told her last night he went to wake up his dad to tell him something and he couldn't get ANY kind of response. He got his grandmother up and she tried waking him up and had no luck. They had to call the paramedics. I still don't know the whole story other than the paramedics were able to get him to respond. He should have never seen this and I don't want him to ever see this again, so my question is how do I go about getting full custody. Right now I just have a document between his parents and I that I have full legal custody and they get him summer (since I lived in another state).