We do demi pirouettes in walk.
When I first did them, I had to make notes after a lesson on how to do them. But do you have poles? e.g. off your jumps?
For a child you could easily make it a game walking in and out of poles.
Our RI lays out four poles, parallel to each other on the ground, like one might have for trotting over? With the ends not far from the school fence.
The exercise is to walk your horse up between the first two poles, turn at the end and walk back between the next two poles, turn at the end (it will be in the other direction this time) and walk back again between the next two poles.
A child will see that to turn the horse sharply enough, it needs a turn on the haunches done without moving farther forward?
One always rides forward into a demi pirouette - it is not done from a stand still. The front half of the horse is blocked from going forwards and its head and neck remain straight with no bend (think rider's back and hands) so, because it cant go forward, instead, the front half of the horse pirouettes round its back legs. It is then facing back where it came from and ready to walk down the next pole.
You immobilise the back end of the horse by putting your outside hip back a little and your outside leg behind the girth. Your inside leg stays on the girth. We use the reins by holding our hands quite close together but moving them ever so lightly to the inside. Leave the shoulders of the horse free to circle round.
I have no idea how a real trainer would teach a pony to do this, but if it were my child I would lead him and the pony round the end of the pole a few times.
In our school I am convinced that the horses know all these games and if you ride them up and down the poles they know absolutely how to turn and go back in the other direction.
It was much harder being grown up and being asked to change reins using a demi pirouette. I ended up with a 563 Word document on demi pirouette. And like your son, I worried a great deal.
But the walking up and down poles game is fun.
The demi pirouette is very good warm up as it brings the hind legs of the horse well under its body - and a boy may thus be able to get a very active walk up and down the poles - with nice impulsion whereas I as an old lady do not.