My soon to be ex husband left the state of CA 3 years ago because his depression kicked back in. He has a history with depression and several attempts at suicide. And not just the kind of attempts you make in order to get attention. He has seen his child twice in that time and has made little to no effort in asking how his child is. Anyhow, his absence created a world of turmoil for our then 3 year old son (who is now almost 7).My child is in therapy and has been for quite some time and his behaviors have been greatly effected by everything. The phone communication went terribly, as in it created a problem for my child, noticeably so that his teachers could pinpoint when the conversations happened and could tell me.So I stopped them a year ago so my child could find a grasp on things and his behavior has increasingly settled. And now after 3 years the ex has decided that he is better and wants to be back in his life.(he has recently become a substitute teacher which is even more scary that schools don't do psych evals) I fear for the safety of my child in his care and knowing what he is capable of doing to himself when the going gets tough, I have a hard time in seeing him as a fit parent. Can his mental instibility be a determining factor in him getting any sort of custody or visitation?

Certainly. But it doesn't completely preclude it. People suffering from depression can be controlled by medication. So a medical eval can clear him for visitation (though probably supervised) and even for a job as a sub teacher (you don't have a clue what research the school did).

Since you said, "soon to be ex", you should be making visitation an issue of the divorce settlement. I hope you have documented the problems caused by his previous contacts.