Do I need to inform my PI about every single project I am working on?
In your comments you mentioned that you are funded by your PI, which changes the dynamics and expectations somewhat between you and your PI; and I think he needs to know how you are spending your time.

My suggestion would be to seek approval from him to work on these. This is to show him that you are respectful and value his opinions on if these are worth working on. It can be in the form of I have some ideas I would like to develop with Prof xxx and Prof yyy, can I go ahead and discuss with them? 

Your PI would probably also need assurances that working with them does not compromise too much in your abilitty to support his lab. And lastly remember this. It is in your best interests that relationships between your advisor and you remain cordial.