After a form (travel expense request) is submitted, I have it forwarded

After a form (travel expense request) is submitted, I have it forwarded to an approving manager. I would like all the fields to be read-only except the one question that allows the yes no approval from the manager.

So basically in editing-after-submission can certain fields be read only?

You can use conditional logic to disable fields when a submission is edited. 

How to Enable/Disable Fields Using Conditional Logic

Inside of the following guide, you can find how to create an approval workflow form:

A General Approach on Building an Approval Process Workflow