What do you do to exercise and mentally stimulate him?
How long is he outside before he starts to dig holes?
I would make sure I am outside with him all the time. When he starts to dig I would 1. distract him with something else to do like playing with a ball, playing with you, do some obedience with him. Ignore the behavior and replace it with something else can eliminate the behavior you don't want but it takes patience and time. I would not let him outside for a long time unattended that is when most dogs get into trouble so they really shouldn't be outside alone unless watched especially young one.
You can also have an area that you allow him to dig and release his instinct to dig and teach him where he can dig like you would teach a dog where they are allowed to go potty (some people do that have a spot).
Dogs dig for fun, to burn off excess energy, if it is hot they will dig and lay in the hole to be cool, or dig make a place for them just to lay, or they dig out of pure boredom.