Going to Court for Custody...what will happen?

My ex and I separated in November of 99 and divorced in July of last year. We have a 3 year old son. I live with my boyfriend and have for the last 2 years. My ex has an extensive criminal record (theft, DWI, countless auto accidents, 2 civil suits, etc...). My boyfriend and I have been together since my son was 10 months old. Since he has been talking, my boyfriend has been "daddy" (not my idea, he just did it one day). The ex, moves around alot, and just last year bought a double wide and moved his pregnant (not his baby) girlfriend in with him. He only knew her for like 6 months. After them being together for a little over a year, they are now married and going for full custody. Their defense is that I am "cohabitating" (living with someone and not being married). Also, my son is HIGHLY allergic to cigarette smoke, and after several doctors notes, I finally had to get a court order for no one to smoke around him. The ex's parents keep my son for the husbands visitation, and they smoke. He has started coming home sick again. Court is soon and I was just wondering what my chances are since I am "cohabitating" and he is married. I feel like I have all this evidence against him, and all he has on me is that I live with my boyfriend. I just need advice from someone who has been there.