What should I do since my cousin was arrested while in possesion of a pistol registered to me?

Felony possession of marijuana and possession of a firearm are serious crimes.  Do you have a permit for the gun?  I would need to know more facts here, but if your cousin was holding the gun as if it were his and had the illegal substance on him, there is not a lot you can do that will reduce the charges.  i suggest that you help your cousin by contributing money with him for a lawyer to represent him in court.  if your cousin has a record the chances of him having these charges reduces to lesser crimes is slim.  nevertheless, there may be a legal basis to have the charges dismissed based on the police officer's procedure of stopping and arresting him.  if you and your cousin cannot come up with enough money for a lawyer then i suggest that your cousin make an application for a public defender.  the PD will assist him and work with the state to make a deal on a plea.  he should however, expect probation with jail time over his head or jail time up to 3 years depending on his record.