My son’s iPhone 12 is switching between apps and acting as though he’s pressing buttons when he isn’t. It’s as though water has got in it but it’s not got wet. The app icons wobble with the line in the corner as though he wants to delete them.

There is likely an issue with the screen on his iPhone. Schedule an appointment at your Apple Store Genius Bar for evaluation and options --> Genius Bar Reservation and Apple Support Options - Apple

It's worth a try, but probably won't help at this point. Perform a Forced Restart on the phone. Follow the instructions very carefully, as this is a bit tricky. It won't hurt to repeat this several times.

Force restart iPhone - Apple Support

There are only 2 possible results when you perform a Forced Restart:

1) The phone returns to normal operation

Or.....if the phone is still experiencing "ghost touches"......

2) There is a hardware problem with the phone that will need attention at the Apple Store (make an appointment) or take the phone to an authorized Apple Service shop.

You can find a list of authorized service shops in your area using the link just below:

Find Locations