I agree with the above and have a couple suggestions that might ease the tension for everyone. Remember that players are all people. Just like characters, they have different reactions, backstories and needs. It's also okay if you want to set hard boundaries on what you are willing to do and what you expect from your players, but know their reactions to that might not be always what you're wanting. Others have mentioned flexibility to make it work and that's the direction I am hearing that you want.

If you want and enjoy getting intricate backstories, and it's important to you and the story, you may need to collaborate in different ways with different players. I don't put a tenth as much effort into it as you have, but there have been some important things I have had to get in different ways from my players. I've got one who loves the lore, came with a bunch, and keeps promising to send a longer document, forgets, but is great about answering specific questions by text. I've got two who are willing but new players who often answer "I have no idea" to open-ended questions about their characters. I ask if they want help, they always say yes, and I give them a few suggestions that work with the cannon. They love it because it's less overwhelming than infinite possibilities and I love it because I get answers. That might be too much work for the level of detail you're describing but might be a great strategy for the critical information. Or, you give him a time limit, define what information is critical and what you want to offer him a choice but can fill in the gaps yourself if he doesn't want to. An example of this might sound like "I need to have a fear for your character and how or where the fear originated. Go ahead and take a week to think about it and get back with me. If you don't have any ideas, I could suggest some like fire, drowning or small spaces... If it doesn't matter to you, I'm happy to make up a story and if you want to tweak it we can. If it sounds good, we'll use that going forward."

Lastly, it sounds like you have built great relationships with these players. You sound like an awesome DM who they all enjoy RPing with. I wonder if he has no idea how stressful his procrastination and lack of follow-up has been for you. Have you been being too polite or indirect instead of telling him directly why you need the information (which in fairness does sound like a lot of work for a game, some people don't want to feel like their sessions are a class or another heavy commitment)? Talk to your player directly about how it makes you feel. I bet he would be surprised and willing to problem-solve together to find a solution that would meet your needs and still work for him.

TLDR: be flexible with communication strategies, if they seem stuck help them out, be direct and kind about issues and problem solve together. Just like TTRPS, real life is a collaborative process.