What are my odds of getting full custody?

My son is 3. He started going with his dad 7 months ago. He visits his dad every other weekend. I left his father b/c he was physically, verbally and mentally abusive towards me. He also has a drinking problem! He drinks everyday!! He also was physical in front of our son when we were together. Which is why we are no longer together. When i left him i filed a pfa b/c of his actions and threats towards me. Since then we have both moved on. I continued to raise my son and take care of him. After about a year he pops into the picture with his new girlfriend and her 2 chidren, and now wanted to start seeing our son. He was granted visitation even with our pfa in affect, dui's on his record and other things as well. He convinced the counselor that he no longer drinks. Which i no for a fact..is not the case. He has come to pick up my son from the daycare provider and she's told me that he has smelled of alcohol. On drop off days i have witnessed him smelling of alcohol. My son has come to me and told me that his daddy drinks alot of beer. He's already been with my family and asked for something to drink. When asked what he wants to drink he says "coors". As i said earlier, my son is 3, too young to know about beer! He's also told me that his dad and the new girlfriend have fights and say bad words to eachother when he is at his dads. He told me "daddy said he's going to put Jen (the new girlfriend) on the streets". Again..not something made up because i myself was told the same thing when i was with him. Now...my 3year old son tells me that his dads girlfriend's children who are 7 and 5, touch his private parts while they are in the tub! When i heard this i was devasted! I also have 2 other children 9 and 5 months and nothing like this has ever happened to me with any of my boys! So i contacted his father via email (b/c of the pfa this is our way of communication) and was told "this doesn't happen in our home, Kick rock"! So pretty much he doens't believe our son! I guess he thinks our 3 year old son fabricated all this! I filed for an emergency modification of our custody agreement and was denied! I called child services and still havn't heard back. Someone that knows this other child who supposidly touched my son, told me that he also touched another child before and that he's very ualy explicite for his age. Even after all this i was still denied. So i had to suck it up and send him back to his dads. It was the looongest weekend of my life! What can i do?? What are my legal odds of getting full custody????