I can see how he forgot. You only hand your phone in, if you know it is in your pocket. If you don't usually take it in and have done so as a one off, and have been revising for the morning, it's quite possible to forget that you have changed your routine, and therefore to watch others hand theirs in, without remembering that for this one exam, you have to do the same.
I am a secondary teacher and yes, I remind every student of these rules every time we prepare for exams, and I remind them why they need to triple check their pockets for every exam. I will remind all of my students again tomorrow, but I know that one day, someone chaotic will forget.
(Actually I got to the end of one of my own GCSEs a hundred years or so ago, reached into my pocket for a handky and felt a wad of revision notes, stuffed into my pocket that I had forgotten. I hadn't used them, had no intention of cheating and wasn't discovered - but even a well motivated, intelligent and scrupulously rigorous in following the rules person like me can have a lapse of memory.)
You sound like a lovely supportive mum. Hopefully your young man will learn from this mistake, be a little more sensible about listening to reminders in future, and go on to appreciate too, that you can make a genuine mistake, but recover and move forward.