My professor wants to know if his recommendation is helpful
You should not be nervous about the enquiry, he just wants to know the outcome. Many academics lose track of time and/or have unrealistic expectations about how fast things move. I would respond that you haven't heard anything yet, but will let him know, regardless of the outcome, when you hear.

If you get in, he will fell happy for you and if you don't get in he will feel a little sad for you. Even if you are unsuccessful, I would not tell a letter writer that the letter was "useless". The only case I might say something like that, and I would be much more positive about it, is if you chose not to use that individual as a reference. Something like: for that position, I was limited on the number of reference I could submit and I felt my other references were a better fit for the position.