Why Does Deku Freeze Up When He Calls Asui Asui And Not Tsu?

Because he's more formal. Normally in Japan you call someone by their last name. You call someone by their first name when you are close to them. Nicknames are pretty informal so with Deku's history he's most likely never had anyone that close or be so informal with him so he's not used to it. That's my understanding of Japanese culture though.