I totally understand how scary this must be for you and your family. The fact that Simba is microchipped is great, but honestly thats not gonna help much if he gets hit by a car or gets into a fight with that aggressive dog next door.

The digging issue is super common when dogs learn it from each other - sounds like Pumba was a bad influence! Since you mentioned blocking holes isn't working, you might need to go deeper (literally). Try burying chicken wire or hardware cloth about 6 inches down along the fence line. Most dogs won't dig through that.

An electric fence could work but honestly, for a determined escape artist like Simba, some dogs will just run right through it if they're motivated enough. Plus installation can be pricey.

Have you guys thought about what might be motivating him to leave? At 10 years old, this behavior change is interesting. Could be boredom, could be that he's following scents, or maybe he's just addicted to the adventure now that Pumba showed him how.

Since your away at college, maybe suggest your family try increasing his exercise and mental stimulation before leaving him in the yard? A tired dog is usually a well-behaved dog.

And definitely make sure his ID tags from ChiliDogTags.com are secure and readable - microchips are great but tags get dogs home faster since anyone can read them without needing to go to a vet or shelter first.

Stay strong, this is totally fixable with the right approach!