Poor lad, he must have been thrilled to be entrusted with the tenner and was overcome with consumer lust and a feeling of wealthy recklessness when he saw the Pringles on the shelf.
I’d have told my own lad aged 15:
1 – You should really ask when you spend other people’s money, as a courtesy.
2 – You should have brought them back to your Mum as a present; she would have said “no thanks I’m on a diet” and then you could have scoffed them with a clean conscience.
He won’t forget the scolding though – I went to the shops when I was about ten and emptied my piggy bank onto the counter to buy an Airfix model but it turned out I was 3 pence short. The shop assistant told me to take the model and come back later so I went to my Mum to ask her for the 3 pence because I knew it was important to pay it straight back. She bollocked me for borrowing money, a lesson I never forgot.
Edit: kids in their early teens are desperate to please and be popular, my own son is currently curling up with embarrassment because while watching the TDF go past he shouted out “Come on Brad!” in a moment of enthusiasm. We keep assuring him that nobody heard but he is mortified anyway.