The phone's "internal SD card" (that's like a floppy hard drive - something that can only exist in a marketing manager's mind) is a USB storage device. But that wouldn't be where the SMS messages are stored. You'd need an app on the phone to undelete them and, as you know, you stand a chance of installing the app right over the one message you need.
I assume that your son didn't back up his SMS messages. That would give you a simple xml file that you could read with Notepad.
And make sure that your attorney points out (by questioning an expert witness - anyone you happen to know who's in the business - that text messages can be selectively deleted, and in this case there are texts tht aren't in evidence - which would put the situation is a whole different light.
(BTW, deleting those messages without his consent constitutes destruction of evidence in a criminal case, which could be felony, depending on where you live - don't let the attorney fail to ask the DA to charge the girl. Then you two fathers can mutually agree to drop the charges against both of your kids.)
You can also check with your carrier's technical people and see if they can recover (with or without a court order) deleted SMSs. An accusation of destroying evidence would make an order pretty easy to get.
And you can try to talk to Google's tech people to see if they can. (SMSs are usually synced to their cloud server.)
In the future have your son download an app that backs up SMSs, and always keep an off-phone backup. Just for situations like this. (You're in IT, so you know the old IBM joke about being backed up, right? ALL backed up because I've been out two days?) You know that any file you don't have backed up to at least two different destinations is a file you don't really need - so now's a good chance to make sure your son sees why.
And good luck with the case. (But unless someone has a hand in someone's pocket, I think this is going to be a "he says, she says - be a good boy and don't let me see you in my court room again" kind of thing.)