W.R.
Coming from a similar situation, I know how frustrating the whole thing can be. What I realized too late was that my daughter was caught in the middle. As much as I thought I was being careful not to have this happen, it did. What you are caught in is the drama triangle. There is always a victim (the step-son), the persecuter (mom and dad exchange these rolls depending on the situation) and the rescuer (also exchanged between both parents.) Son gets bad grades, he feels like the victim (teenagers have problems being accountable and responsible) because he gets in trouble. Persecuter comes along and grounds the kid. Rescuer delivers him to his friends. The only way to get out of the situation is break the cycle. As a persecuter your husband can start making agreements with the son with clear cut consequences. For example, sit down make a behavior contract. Make him make agreements with clear cut prices to pay that you both negotiate. This will allow him to see that his behavior will shut down priviledges, no persecuter. Words like this, "You made an agreement to do your homework before 8pm. Well now you don't get to go on the computer, that was our agreement." Now the natural consequences kick in and no getting angry and upset when the report card comes in with low grades. The drama triangle will continue until someone steps out.
Good Luck,
Wendy