Professor didn't write a letter because I didn't waive my right to view it. How to let him know that I have changed this? I'm applying to UIUC and have asked 3 of my professors to write me recommendation letters. One professor, who I discussed my application with before extensively, wrote back to me after I was done with everything that he doesn't write a LOR to students who do not waive their right to view the letter at a later date, because if I don't trust my professors, I shouldn't be asking for their help. 

I'm pretty dumbfounded by this, considering that he never mentioned this before and that I've never come across a situation like this. It's not even a trust issue! It's pure feedback. Why would you keep feedback from someone anyway?

So I waived my right to view his letter, but now I'm not sure how to write back to him. I don't want to write an uncomfortable email to the department I'm applying to - UIUC procedure - as to why I have to switch professors if he still refuses to write one. Is there a polite, apologetic way to say to him that I had not intended to convey any feelings of mistrust, and that it was to see what were my strengths and weaknesses from a professor's perspective? Or should I take the weird route and change professors, because he'll write me something less-than-glowing now?
Just simply write down that it was your mistake not waiving your right to read LOR and ask him to write a letter. Don't wait too long to respond back to him, make your response succinct. Most importantly, do not explain to him why you wanted to see the letter in the email. Good luck.