Fast forward to today. Having almost burnt the house down the first time - lesson to be learnt here - using a silicone tray from a cheapy shop (lesson: never trust them!), I decided to use my old muffin tray lined with foil instead. Then once they had melted, I let them cool down enough so I could handle them. I then shaped them into 'rocks'. This way he can't use a palm grip as they fit neatly inside his palm, he has no option but to use a pincer grip.
Now while they don't look pretty, they do serve their purpose. I just used cheap crayons from the Warehouse because every $2 type shop had been cleared of crayons and when I asked at the counter they looked at me like I was asking for something rare!
Demonstrating a pincer grip
Left and ...
Find Lou's tutorial here