Puppet Master

I think it would be a good idea to sit down with your son's mom and have a frank discussion about your worries. Don't pressure her into making a decision, but tell her that you are concerned about Steve's mental stability. As a father, it is your job to ensure the safety of your child. I'm sure she feels the same and would not put your son in danger just to be on good terms with an ex-boyfriend. Ask her (nicely) what makes her think that Steve is now considered stable enough to be around your son.

She might say that it was just a one-time thing, but anyone who stabs themselves over rejection has much deeper emotional and psychological issues. Unless he is receiving treatment/medication/therapy, then it is possible that he can be triggered and have another breakdown. Have her promise not to leave the two of them alone, even for a few minutes.

I have to add that you should talk, not yell. Since you've already been through what sounds like a messy break-up, I'm guessing the two of you aren't very friendly. However, you'll have to put any negative feelings for her aside when you are discussing your son, otherwise you might lose the opportunity.