Yes, there is no requirement that someone be charged the day or right after the drugs were found, or that the charge or arrest be by the same law enforcement agency. Five months is well within the time frame for bringing criminal charges, so based on what you write, the authorities could arrest him months later for this. However, it's possible that the search was not proper--a seatbelt violation does not necessarily, without more, lead to the right to generally search the car--so it may be that the search can be surpressed and the drugs thrown out as evidence you and your son should speak with a criminal defense attorney, not just generally anytime you are charged with drugs, you should get a lawyer but specifically about the facts of the search.