if the structure is going to be staying in the same spot you would be able to dig it into a depressed area and just use the existing soil poured back into it, you wouldnt be able to use the wood though unless treated but that would ensure that if he liked to dig it would not let him escape, the natural fauna and flora would be able to come and go as it pleased, and would still be relatively soft in case of falls. I think the better choice to protect him though would be to make the climbing areas as wide and stable as possible, maybe even making a second level within the structure that is accesible through a ramp so it is even shorter a distance if a fall takes place.