Usability Testing a website redesign

My company is working on an agile website redesign project. The website is a content-focused, lead gen website. We have come to a disagreement internally about when to perform usability testing. I think we should do it along the way with what we design in a sprint. A colleague thinks we can't do that because we won't have all of the user flow pages designed with the full copy to test with. He believes that to test user flows we need full copy and therefore won't be able to test until the project is nearly done. Does anyone have any insights on how to perform usability testing for a content-heavy site for lead gen?

Thanks so much!