I'm sending a draft to my coauthor (senior and more experienced). I'm new to the world of coauthoring and was wondering what would be a good way to suggest that he has absolute liberty to change anything and I would not be offended by that. I feel awkward to say this directly, because it can imply an assumption on my part that he couldn't do that without me telling him.
Based on my 25+ years of academic co-authorship: Your co-author definitely won't feel free to change everything unless you tell him so directly. Don't feel awkward. Some people fall in love with their first draft and only want the co-author to "improve" it but not "change" it. If it's a really good first draft that's not so bad, as it will speed publication. But usually, the first draft needs changing, not just improving. And sometimes serious changing. So let your co-author know you're not one of those types. Your wording sounds good to me.