Sons father and his drug addict girlfriend.

Hi there,
So kind of a long story, my ex has been in and out of my sons life since he was born (he's 3) we broke up when I was 9 months pregnant due to his verbal abuse becoming physical. Anyway, he comes and goes, he has recently come back after no contact for 4 months. He has a 10 month old with another women, I'm constantly being told (by him) about their relationship problems (i tell him I want no contact unless it involves our son, but it doesn't stop him, even if I don't reply or one word replies) any who, about a month ago he informs me that he now has his 10 month old full time as the mother has picked up and left, I thought it was a bit odd, but as usual, I tried not to get involved in his personal life. A few days ago he messaged me saying his "ex girlfriend" was on meth and threatening him, that he was getting a restraining order and I shouldn't send my son there because of her being unpredictable, (he also said she threatened to tell me that he is abusive to my son while my sons there, I honestly don't even know how to get into that can of worms) Now yesterday I get another message saying she came over and talked and now "everything all good" and when I told him I still didn't feel comfortable with my son being around a hard drug user, I was told I was being a bitch, that I couldn't keep him from his son and he didn't want to fight, then I was blocked on facebook (our only form of contact as he has no phone). I really do not want to send my son there, but I feel extremely guilty that my son will miss his dad. I also don't know what will come of this, or if I will ever feel comfortable with him seeing his father, (as they have a kid,and she will always be a part of their lives) I also don't want my beautiful son to have to deal with this drama for the rest of his life! I have done so much to try and provide him with a stable household and shield him from all the crazy.

Has anyone else felt with this situation? What's the best course of action? Supervised visits? Paperwork of when she will be there(to ensure my sons not there at the same time) Or should I just leave it? Or even file for full custody? I'm so confused, I just want what's best for my child.