It sounds like you are worried that he (or someone) might scam you.
There is no special legal status for an editor (beta or otherwise) that would protect your ideas or actual writing more than from anyone else. Sharing something with a reputable editor or agent does mean you can rely on, well, their reputation, but there is no additional layer of legal protection. You have a contract once you sign with an agent or publisher, but they don't add any protections to your copyright (they actually decrease the protection of your copyright, in exchange for money).
If you're worried about him stealing your work, maintain good records documenting what and when you shared with him (e.g., an easily accessible email thread). If you're worried about him sharing your work with others who will steal it, ask him not to share it with anyone without your permission.
If you're worried about him stealing your ideas, don't share them with him. Your ideas have no legal protection. Alternatively, stop being worried about people stealing your ideas. For this to harm you, they would have to develop, package, market, and sell your idea. 396
In this circumstance, they're the one doing most of the work, and by the time the final product is sold, it's something different than what you would have done with it anyway.
The only benefit to formalizing your arrangement with a beta reader, then, is so that you have the same expectations about your relationship. That's an interpersonal issue. Decide what you expect from him (e.g., regular notes, not sharing your work with others, or maybe even the opposite -- finding other people who would want to read it) and what you're willing to give (e.g., drafts of your work, maybe returning the favor and reading his, thank you gift(s), or a mention in acknowledgements). Ask him what he expects from you and what he's willing to give. Then come to an agreement together. If you'd like, you can put the agreement in an email so you both can refer back to it. You might bring this up by telling him you've really appreciated his ideas and suggestions, you hope he's enjoyed reading your drafts, and you'd like to have a conversation about formalizing the arrangement. 487