What are my chances of getting sole custody?

I am serving my ex of my 8 year old son with custody papers asking for sole physical and legal custody as well as child support. I am nervous that he is going to contest it only to spite me, not because he really wants anything to do with him and that this will turn into an ugly, expensive battle. Does anyone have any advice on how I can prove my complaint of neglect. He has not had any contact with my son for over three months. He even promised to take him Christmas shopping and never showed up or called to follow through. Even before all this he would only get him maybe once a month for a few hours on his terms, late picking him up and wanting to drop him off at the drop of a hat (no notice). I have word that he is doing steroids and selling them as well as prescription narcotics, problem is, you have to be able to prove that in court and I can't unless having a witness is good enough. I provide full support for our son including health insurance and dental and I have multiple bills racked up for health expenses that I have been paying on for years from before I was able to pull out this individual insurance for he and I. I'm not trying to keep him completely away from his child but I think based on empty promises and neglect to be a part of his life as well as financial neglect its in the best interest for him to be in my sole custody, not to mention we have no custody established at all right now and it needs to be done. Do I have grounds for sole legal and physical custody and how do I support my case?