R.K.
Your son is fairly young and easily scared. No matter what he loves his dad and wants to spend time with him. Kids are very forgiving. I am not saying your ex is in the right here, but looking at it from your son's point of veiw.
Your ex has been court ordered not to drink when he has your son. Get out the court order and keep it handy. Next time your son calls and tells you dad is drinking, call the police and ask them to meet you at his home. Take a copy of the court order with you and show it to the officer. Ask the officer to remove your son and his belongings from the home. Now you have documentation in the form of a police report to take to court with you. For as long as it takes file a police report EVERY time you are told he is drinking while he has your son. Yes this is a hassle, and a pain, but do it.
Hopefully you can get a court order to make your ex go into rehab and start AA in order to continue seeing your son.