Custody fight over 14yo son

I am the mother of a 14yo son who is now stuck in the middle of a custody fight. My ex decided to do drugs and have an affair 10 years ago so we divorced. He has since lived with and been involved with 3-4 other women. He has never been alone and just leaves one woman's house and moves right in with a new one. He married one of his coworkers about 16 months ago and this allows for a much better financial situation for him. He has decided he wants custody of our 14 1/2 year old son. My son has made it very clear he does not want to live with his dad but he continues to pursue this. He said that it's because of my sons grades. My sons grades aren't great but they aren't terrible either. However, my sons grades do not improve when he is with dad and I have teachers notify me of behavioral issues when he is with his dad. We had mediation last week and I didn't have an attorney. We agreed that we would alternate custody from week to week and have a Guardian Ad Litem assigned. My son has epilepsy, ADHD and tremors. His dad has not been involved in his medical care at all, well at least 95% uninvolved. He hasn't had interaction with teachers or school staff until this school year. I have no past that can hurt me, my son is well taken care of and wants to stay with me. I am speaking with an attorney tomorrow. What are the odds that his dad could really get him from me? How do I deal with the fact that his dad hasn't been involved and now all of a sudden he isn't making it a point to be involved and look like super dad?