There is a program in the Windows 10 store called WP Message Backup which will bring your text message down to your PC. It works although it is not optimal because it brings all previous message down and I seem to be having an issue deleting the unwanted messages but they are all there so you would have your son's messages. It is worth checking out in my opinion.
https://www.microsoft.com/en-us/p/wp-message-backup/9nblggh2srtp?activetab=pivot%3Arelatedtab
Also, I tried this and it worked but secondary attempts I've had issues with although I'm probably doing something wrong. It works fine for textx but in trying to send MMS, it worked initially but future attempts are failing to accept the email address. (Update: So I was able to send a few more MMS text messages and they went through. So it is not perfect but is a way to do it particularly if you want it in email format to save or forward)
I'm not sure this is the best option but it will work for texts. I just tried it and was able to get it to work. I created a new contact and used my email address as the mobile number. Then did the following:
Step 1
Open the "Contacts" list on your cell phone. Enter a new contact, listing the email address where you want your text message to be sent. Give it a name you can easily identify to avoid sending the message to the wrong contact. Click "OK" and check to see if the address is listed in your contacts.
Step 2
Open the text message in your inbox. Open the "Options" for the message and select "Forward." Select the contact you created in Step 1 and click "Send."
Step 3
Check your email inbox. Keep in mind that the message may not arrive immediately, but if it has still not arrived after a few hours, attempt to send it again. Depending on your cell phone provider, the message may be delivered as an attachment or as text within the email.
Step 4
Download the text message attachment and click "Save" when the download pop-up appears. Create a name for the file and save it as a text file, like a .doc or .txt. If the message was sent as text only in the e-mail, copy and paste the text into a text editor like Notepad or Word. Click "File," and then "Save as." Give the file a name when the "Save" pop-up appears. Press "Save." Your text message is now on your cell phone, in your email inbox and saved as a document on your computer.
Via: https://www.techwalla.com/articles/how-to-save-text-messages-from-a-phone-to-computer