Express sympathy for a person labeled as a felon for sending explicit text messages, and argue against the severity of the punishment. Also suggest that the power dynamic in relationships with large age disparities can be complex and that it is not always clear who is at fault.
I don't know if you know this guy personally because it seems like there is something personal about this to you, but being labeled a felon is very serious and follows you your entire life with no way to ever make it go away. What he did was VERY stupid but I don't think that sexting with someone is worth prison time, shunning from your family, needing to inform your neighbors you are a sex offender and the inability to find a job. Part of the problem with 18 year olds being in high school with 14 year olds is the likliehood that this will occur, and it does all the time without being reported. It's easy to delete messages and make someone look in the wrong and generally speaking the girl is believed above the guy because no one wants to make it seem they don't think rape or coercion is a big deal. I would be willing to bet there was something mutual going on here. You do not get a persons phone number and send them messages like that without some idea that they will be reciprocated.