Okay, here's the thing with digging. It could be many reasons why he just started up again. First, is he neutered? If in-season females are around that is sure to spur that wanderlust. Even if that is not the case...Siberians are notorious for digging to China.

If he is just digging to ease boredom then providing him with plenty of daily activity and mental stimulation is needed. Stuffed Kongs, Buster Cubes etc. Plenty of exercise.
This could be and "escaping" issue or a "digging" issue, I'm not sure if you are concerned with both or not.
Has anything changed recently in your household or in the neighborhood? That could bring on a change in behavior too.

For his safety you must ensure that he remains contained in the yard. Many people dig a trench along their fences and embed some wire mesh to prevent escapes.

If he is just digging for the fun of it, consider building him a sandbox and encourage digging in there by burying toys and treats etc... It's easier to redirect this kind of behavior than to completely eliminate it.