he can't use sand unless he changes the filter, which sounds like an undergravel filter. Bit hard when you have parents to answer to and who will ultimately doing the spending, so I can understand where you're coming from - but for the health/well being of your axie, you really need to get your parents to read the threads you've posted. Both gravel and undergravel filters aren't the best things to have. UG filters are probably ok for fish, but they don't swallow gravel - also a lot of waste/gunk builds up beneath the filter plate.
With sand you need to regularly stir areas of the sand, so pockets of gas don't build up, an axie also will snuffle through the sand if it thinks it can smell food (ie worm burying itself in sand.) and may stir up enough to get to the gravel layer; so I'd say no.
Can you get large pieces of flat slate - this may work better. If you remove the gravel and enough pieces of slate together so they cover the tank . floor.