13-1405. Sexual conduct with a minor; classifications
A. A person commits sexual conduct with a minor by intentionally or knowingly engaging in sexual intercourse or oral sexual contact with any person who is under eighteen years of age.
B. Sexual conduct with a minor who is under fifteen years of age is a class 2 felony and is punishable pursuant to section 13-604.01. Sexual conduct with a minor who is at least fifteen years of age is a class 6 felony.
13-1407
F. It is a defense to prosecution pursuant to section 13-1405 if the victim is of the age of fifteen, sixteen or seventeen, the defendant is less than nineteen years of age or attending high school and is no more than twenty-four months older than the victim and the conduct is consensual.
While it is a DEFENSE to prosecution that the two are so close in age and that your son is 18, it doesn't mean that he can't be arrested and prosecuted for this...