Stepsons mother allows him to drink beer, what can we do?

There's not much you can do about him drinking at his mother's. The best you can do is tell him that he is not aloud to drink or come drunk to your house after his mother's house. You may have a possible chance of custody. Go to court and show them pictures of how the kids look when they come to your house. Show the proof of difference in grades and effort of when your stepson moved in with you.