Son would like a refund, please

My 11 year old son got a laptop for Christmas. He set up a bank account for his Christmas money. 

Thrilled with the possibilities of this he signed up for a free trial of Adobe Photoshop. Six days later he decided he didn't want it and cancelled it.

Turns out he didn't cancel it right and his bank account was debited by £19.97. That's quite a lot when your pocket money is £3.50 a week. 

He insists he cancelled Photoshop but Adobe doesn't seem to think so. He's now worried that further instalments will be taken out of his account despite his bank assuring him it will not happen. 

Given Adobe has assets of almost $20 billion it would be a nice gesture if they could accept this as a youthful mistake and refund my son his money.