Letter of Recommendation Reminder (deadline tomorrow) I asked my professor to write a letter of recommendation in late July, with the deadline of October 1st (which is tomorrow). He responded in the affirmative, and was quite happy to write my LOR. I sent him a reminder email a week ago, which he did not respond to. The deadline is tomorrow, and I still haven't received any notice that he uploaded his LOR.

The other professor I asked already uploaded his LOR, so I'm just waiting on this professor. At this point, unless he uploads the LOR on the deadline, I'm afraid it will be late. 

How should I send him an email to prompt him to upload his LOR? I already graduated so I can't visit him in person. Is there a chance he's backing out of writing one for me?

Thanks for any advice in advance!
With a deadline of tomorrow I'd say it's too late to email him. Phone him instead. Try to pick a convenient time (accounting for timezone differences). If you can't reach him, call the department office and leave a message.

If he's already committed to writing a LOR for you, it's unlikely he's backing out now. Yes, he might have underestimated the amount of work he has left or procrastinated a little too much (don't we all), but he'll be well aware that you've put your faith in him to write, and if he refuses to write one now you'll almost surely miss the deadline (and by extension fail to get admitted). It's only responsible for him to finish the letter then.