Just say any of these: 1. Thank you for your assistance. 2. Thank you. I appreciate your offer to help me. 3. Thank you for responding. I woyod appreciate your help. 4. Thank you. I'm looking forward to your help. 5. Thank you for offering to help me. I really appreciate it.

"I really appreciate that! Thank you." By the way, "that sounds good" is not a natural way to respond to an offer of help even informally, as it lacks feeling on an interpersonal level. Informally you could say "I'm so happy that you said that"

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.

In the United States, "That sounds good, thank you!" would be acceptable in an email to practically anybody. I don't think there's any need to be formal. You're communicating agreement, you're communicating thanks, it's the thought that counts. I can't think of a real stock phrase that would be slightly more formal; perhaps "Thank you very much for your kind offer; I gratefully accept."

For a more formal version of sounds good or for recipients that have English as a second language I use: That will be fine.