Well, I took myself to mountain warehouse, and can confirm to all those who read and wondered what the answer would be: A 10 year old needs an adult sleeping bag. There is a sale on, so he got himself a super light weight 1.4kg adult size sleeping bag at 215 cm (not 230) season 3 bag, for £34. I am well pleased.
I dont want him to get an adult one just yet, but he has outgrown his Gerbert kids bag.
The reason why I dont want him to get an adult bag is that WEIGHT is of the essence, as it needs to be carried for around 10km per day, when trekking through the mountains. We have too much already with change of clothes and food for days on end....
So, lightweight, small, packable, and portable for a 10 year old boy is of the essence!!