I think that might confuse him more tbh. He's learning the basics and I think you need to understand interfaces to understand dependency injection.
- You can inject another object that implements that same interface for unit testing. So for testing instead of using a database repository object/ web service you can inject your own dummy object that implements that returns dummy data
- It's very easy to pass objects around using its interface so if you change to a different object you don't have to change all the references e.g. lots of collection objects implement IEnumerable.