In most state, a person must have effective custody and control over the drugs in order to be in "possession". A passenger in a car is usually deemed to have constuctive possession.
The only way to avoid prosecution in this situation is for the driver to fully admit that the drugs were his, that the passenger had nothing to do with it, and the passenger did not know they were in the car. At some point (with a lawyer), your boyfriend will have to explain the situation.
Here's the problem, prosecutors are reluctant to believe these stories, because they assume it's just one party to a crime trying top help out a buddy. Both versions will have to make sense given the circumstances.
All of this would impact sentencing greatly, especially if the charges are dropped or reduced.
Your boyfrind will have to discuss all of this with his attorney. He absolutely should not talk about it with anyone else, not the police, no one other than his attorney. If at all possible, I would recommend that you work towards hiring a good private attorney that has experience in the court that your boyfriend will appear. Those relationships are invaluable.
I hope this helps.