For anyone who is following this post, I have managed to find a replacement rear wheel for my sons Oset 16 bike. Please see the link below, it has already made a difference to his riding and doesn't spin up as easy, I also fitted a rim lock at the same time and run it with no pressure in the tyre.
He has also competed in some competitions and one has really mentioned owt about ACU tyre rules.
https://www.dirtbikebitz.com/collections/motocross-enduro-tyres/products/gibson-mx41-mud-hardintermediate-motorcross-tyre-80100-rear-p-81190