Predictions are tough in family law. What I can say is that the adjudication process of a 610 petition (A modification of custody) is even more difficult than an initial custody proceeding - especially in Cook County.
If money is tight, contact legal aid for assistance. Otherwise I would highly recommend retaining an attorney. Your case will live and die on how well you conduct discovery, along with any recommendations from the Child Rep/GAL and/or 604b evaluator. In many cases, the pretrial is the "make or break" for most disputes, and you'll want your ducks lined up in advance.
Again, it is an amazingly complicated process. Talk to an attorney in your area for specific guidance. Best of luck.
Honestly? Not terrific. Your specific case definitely shows that he is not parenting in the right manner that he should be, but absent some actual uncontroverted evidence showing that he gave him a beer (besides a he said she said situation), coming back in to change custody is not easy. Be prepared for a long expensive battle. I'm not telling you it can't be done, but be prepared for big-time fight. And also know that if you have no evidence, your word is not good enough, unfortunately.
This should not be construed as legal advice or the making of an attorney-client relationship. this information is purely for informational purposes and should be treated as such.