You can also use services like https://transferwise.com/ or Send Money Abroad. Online Global Money Transfers | Azimo - they should offer a lot better rates and smaller fee. You need to send it to following details AND include your son’s Monzo 9-digit number (it’s on the card):
(click small arrow on the right to see better formatting)
You can send money via bank transfer or simply pay with your card, very convenient. You will not need your son’s bic/ibans.