My heeler/boxer dog is destructive during the day when we are at work. He digs holes, chews things. How can I fix it?

Increase exercise. I would suggest at least an hour exercise for him.
Crate him when you are gone

I agree with Ruckus (good to see you again btw) exercise and mental stimulation is the key with a destructive dog.

If you already have plenty of toys and things for him to do while you are out have you considered crating (providing it is not more than a few hours) or confining to a single room?

Separation anxiety is another thing that can cause a dog to become destructive.

Yup some good behaviour modification, and protocols for relaxation will help a lot as well.
(Are you guys tired of hearing me say that? )
Like suggested, you might want to consider crating, this well help with separation anxiety, and save your house, and be the safest place for pooch.
Here is a great web site to get you started at home for the relaxation protocols.

Protocol for relaxation (http://dogscouts.org/Protocol_for_relaxation.html)

Believe me it works. I have a dog who suffers from separation anxiety as well. Not fun.

Here is some info on crating your dog.

Crating Your Dog: Is It Cruel to Confine Dogs? (http://dogs.suite101.com/article.cfm/crating_your_dog)

In my opinion crating or baracading ( in your case crating) is the safest place for a destructive dog. It is like a den for them which brings them comfort. Make sure to not use the crate for punishment, this needs to be a safe, happy place for your dog.
I like the seccond web site, it has all the basics of crating, and why you should crate.
Crating Your Dog (http://www.k9web.com/dog-faqs/crating.html)

Sorry if this is all over the place, I am running on little - no sleep. :)

I second protocol for relaxation! Great idea.