If he is helping you with something, it is fine to say " Okay, that sounds great! Thank you." You dont have to make everything sound "super intelligent". I live in America and thats how I talk, in emails, to my professors. Don't stress too much about formal and informal because as long as you aren't using slang of ebonics you are fine. If you are clear and understandable (which you seem to be judging by the paragraph you wrote) that is the key to learning a language.