Word or phrase for tasking an expert with jobs that a novice could do

Busywork is a general term for this, but doesn't quite capture the disparity between his abilities and assignment. Link-MW

The military has many colorful, pithy expressions for this sort of thing. "Counting dolphins", "piloting a desk", "interrogating the snow", etc.

I agree with @Linl-MW. You could also say he is overqualified for his work. Or that he is stuck at a backwater station.

If someone is put out to pasture, he is forced give up some responsibilities.

Example: Have any of you ever seen a manager put out to pasture in a company? In such a state of uselessness, he creates makework for himself and others, and becomes a very petty person to work with.

In some professions, to be given a desk job or turned into a paper pusher indicates that the person has been removed from their area of interest and expertise and assigned to a relatively mindless task, and they are no longer "out in the field" and being productive.

I used to run a pharmacology lab, but they've made me a paper pusher.