I personaly believe that the above argument is rubbish, trainer kite do not have depower.
There is nothing to learn from them,flying a kite is not difficult, you pull left and it turns left, you pull right and it turns right.
There are no skill in flying a trainer kite.
It makes money for the retailers and it is a good toy for the kids but pretty useless as a mean to learn kitesurfing.
You may get a vague idea of the wind window and realise where the power is but it takes 5 secs to work that out on any kite.
Lessons however are necessary as they are are many safety issues to take into consideration and the advice of an experience teacher who can assist your son is extremely valuable.
Lessons will also be conducted on a real kite that doesn't zoom around like a mosquito after a red bull bath, he will need to learn about positioning, launching, powering,depowering, safety release, rigging, self rescue.....
Money well spent when it is your own flesh and blood