If you care for yet another take - mustn’t it be very stressful for him to adapt his word count needs to your gig limit ;), don’t you do custom offers for the exact word count people have? Maybe he’d relax a little if he knew he could order exactly 857 or 1012 words or something for the price relative to your 1000 words gig, so he’d not feel the need to watch his word count like a hawk and wouldn’t feel like underpaying or overpaying?
As for the chasing, you could try to not let him wind you up (I know, easier said than done but you know how they say it always takes two), just calmly assert your own control over your part of the deal, maybe save a quick response like “No worries, you’ll receive your file within the delivery time”; maybe he either needs an additional “ritual” to feel reassured and if it just takes one click more maybe it’s not too bothersome for you, or maybe he’ll realize that there is really no need to worry eventually.
“Why do some people feel the need to do this?!” vs “but I think he is mean” sounds a bit contradictory to me; if you see the possibility of him “needing” to do that, it probably means he isn’t mean but just has some issues.
That doesn’t make the situation itself much better for you, but I find it makes it easier to not get wound up if one acknowledges that some people have more issues than others (don’t we all have some) and to just accept it if it just means one additional message per order for the people who need it (if someone really disturbs your workflow, it might be worth a bit of thought, of course, or might not be worth the pain and time ultimately, we’re not being paid for being customers’ personal psychologists, after all, at least most of us aren’t).
As long as he keeps ordering and pays the correct price (with the 999 words balancing out the 1001 words in the grand scheme of things I admit I rather find this funny, actually) and needs one additional message to feel at peace, whatever. If he’s neither abusive nor a scammer, nor do you feel he isn’t rude but just a pain (maybe just decide to not let it be a pain, it might work), I wouldn’t call that mean, mean would be if he kept leaving 3* star reviews and coming back ;).
Listen to your gut feeling is often good advice, however, sometimes, it can also be good to face one’s demons ;), or to go beyond one’s comfort zone, sometimes it’s really best to part ways, sometimes it’s an opportunity to work on ourselves and might turn out just fine, you’ll find the way to deal with it that’s just right for you at this time.