My son's iPhone 12 plus won't turn on or off yet the alarm won't stop ringing. He cannot slide up to open or power off using the side button

Force restart the phone:

• Press and quickly release VOLUME UP
• Press and quickly release VOLUME DOWN
• Hold the SIDE button until an Apple logo appears (15-30 seconds), ignoring "slide to power off"

The operative word here is QUICKLY; and try it a couple of times, the timing is a little tricky.