Hi
Maybe the sand pit might keep encouraging him to dig? Depends if you want him to keep digging or not?
One of my dogs Daisy a JRT was a really bad digger when she was young, she even dug a hole in the lounge carpet right through to the floor boards which was directly in the middle of the room
She also dug a hole through the carpet in the boot of the car through to the spare wheel and used to dig holes in my garden.
I fenced off the grass in my garden and constantly discouraged her from digging with various toy's. It also helped when I got another dog. She is 5 now and it never enters her head to dig anymore thank goodness.
Cazzie