Ask HN: How to best mentor a new grad?

I have a few years of experience out of grad school and really enjoy teaching and learning. We recently hired a junior engineer fresh out of his master's and he's really new to software development in industry. He's been on the team for a week and I've taken the time to teach him some stuff and gave him some good overviews. We left off with me recommending he read the codebase for a few days to ask questions and understand the flow. My question is, is there a better way to get him up to speed? We use a huge technology stack and he's unfamiliar with all of it, so trying to figure out how to help him out the best way.