Your boyfriend is presumed innocent. Under the law, if he did not know the blunt was in his truck he was never lawfully in possession of it. Possession means you know the item exists and you have the intent and the ability to exercise dominion and control over it. I would advise your boyfriend to fight the case and to convince his friend to do the right thing and testify in court that he had smoked the blunt and tossed it in the truck and your bf had no knowledge of this. Good luck and let me know how things turn out.
In some states possession of marijuan can cause a license suspension, even if the person is not driving. However, this is a case where the "equal access" doctrine may apply. If a person has access to the space where the drugs were fund and your boyfriend can show this, he might be able to present a great argument to a judge or jury during a trial, that someone else had possession of the marijuana. The state must prove that boyfriend knew the pot was in the car. If I put a joint in his pocket and he did not know, then he should not be guilty of the charge, the same thing applies here.