Hey Jamie,

I think the first step in controlling the problem is to figure out what exactly is wrong. I agree that with your assessment that it could be spider mites. You can test for the presence of spider mites by placing a white paper underneath the tree, and then shaking the tree. Take a close look at the paper and see if you can see any small arthropods moving around. If this is a spider mite problem, you can apply a miticide from a local nursery.

A second possibility is that this tree is struggling due to lack of water. I am not sure where you are, but where I am in Northern Kentucky, we have had very little water these past few months. If that is the case for you, your tree might be defoliating due to lack of water. If you've had dry weather, I would make sure that you are watering a newly planted tree such as this one. 

It is normal for an evergreen to lose some needles. However, an evergreen losing this many needles could very well be a big reason for concern.

I hope this helps!

Robby