Originally Posted by: wongywongytoo

Thank you so much, everyone.

You're welcome! ddidler mentioned exercises that can help. This series of exercises is not aimed at your son's problem. But it does show how to use alternate fingerings in order to build strength & work around problems in a way that might be beneficial your son's situation.

https://www.guitartricks.com/tutorial.php?input=1292

If he has problems with using his middle finger in some situations, then he might be able to develop workarounds with his other 3 fingers. Best of success!