Lets see... your son is 19, she is 15 (the 1/2 makes NO difference and is clearly added in an attempt to 'enhance' the age). As such, she simply has NO right to give consent.... and your son is expected to act like an adult.
Your post doesn't say EXACTLY what he was charged with, though I assume it was 'Aggravated Indecent Liberties with a Child' (§21-3504)
"Aggravated indecent liberties with a child. (a) Aggravated indecent liberties with a child is:
(1) Sexual intercourse with a child who is 14 or more years of age but less than 16 years of age;
(2) engaging in any of the following acts with a child who is 14 or more years of age but less than 16 years of age and who does not consent thereto:
(A) Any lewd fondling or touching of the person of either the child or the offender, done or submitted to with the intent to arouse or satisfy the sexual desires of either the child or the offender, or both; or
(B) causing the child to engage in any lewd fondling or touching of the person of another with the intent to arouse or satisfy the sexual desires of the child, the offender or another; or
(3) engaging in any of the following acts with a child who is under 14 years of age:
(A) Any lewd fondling or touching of the person of either the child or the offender, done or submitted to with the intent to arouse or to satisfy the sexual desires of either the child or the offender, or both; or
(B) soliciting the child to engage in any lewd fondling or touching of the person of another with the intent to arouse or satisfy the sexual desires of the child, the offender or another.
(b) It shall be a defense to a prosecution of aggravated indecent liberties with a child as provided in subsection (a)(1), (a)(2)(A) and (a)(3)(A) that the child was married to the accused at the time of the offense.
(c) Aggravated indecent liberties with a child as described in subsections (a)(1) and (a)(3) is a severity level 3, person felony. Aggravated indecent liberties with a child as described in subsection (a)(2) is a severity level 4, person felony."
The fact that she is over 14 and consented precludes the charge of rape (§21-3502).