the instructor at your local kite school is the one to ask this question

titans said...

my son is 11yrs old and wants to kite his weight is 40kg can someone help me with the size of kite he should be learning on please

Cmon dad you just want a highwind kite, catalyst or reo insane depower, plenty of schools teach on the catalyst, no better advertisement imo, would wind to weight ratio work , I'm 85kgs and can fly the cat 8 in 40plus,so at 40kg, you'd think he'd b good at 18knts

^^^^^^
Im their dad.
Titan be careful with your choice of kite, most small kites are designed to fly in strong winds. What you need for your son is a kite that flies well in light winds. Get one that is light, does not luff easy and will sit still and turn well in light winds. I got the park because it has only 3 battens and this keeps it light. As kitersmini says you will also need a kite where the depower is all in the first few inches of bar movement because 10 year olds cant push the bar out very far.
Hope this helps