Possession of Marijuana Charges After a Traffic Stop

My question involves criminal law for the state of: michigan. On april 02 of this year. My son, two nephews and a friend went out to the club. At 3:30 in the morning they got pulled over. My 18 year old nephew was driving. The found less than a gram of marijuana in his cigarette pack, which he admitted was his. They breathalized all four boys. My son and nephew that was driving blew 00s. The boys in the back seat blew bad, they got MIP tickets. They arrested my 18 year old nephew for MIP, possession of marijuana and driving under the influence of a controlled substance. Because the boys were always told to be honest with police, they admitted taking a hit around 9pm. They let my son go without any citations. I just got a letter in the mail yesterday for notice to appear because they are charging my son with possession of marijuana! Is this legal?

If you're asking whether they can cite him after the fact, of course they can. They did and there's absolutely nothing improper about it.
Get your son an attorney.