solved
Try your other options first, imo
This is called the “soft no” dude. Nobody likes to come right out and say no anymore, but I get the impression this prof wants you to go elsewhere. I’d oblige him.
This. Get someone else duder
You don’t really need a professor you’ve taken multiple or upper-level courses from. They just need to like you enough to make a semi-personalized letter about you. Sometimes all you need is a kind-hearted professor. Email around some more, even to longshots. Also, volunteer to come visit them in office hours to discuss things. Face-to-face gives you better odds, it might jog their memory of you a bit more, and you can give them a good impression of you again.
I agree with all the previous posts. You’d be better off choosing someone else that can write you a thorough letter.
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Do you at least know a TA or something like that who could write you a letter? LoRs as it is don’t really do much to overtly help you in the admissions game and are, as I understand, mostly a neutral factor. But I think if you got a letter that basically said “I really don’t remember this guy but my gradebook says he did well in the class” that’s going to raise a red flag in admissions and, instead of doing nothing to help you, will very likely hurt you. I’d try to find a TA or some professor that (despite maybe not having prior work from him) remembers you or something
Only having one academic letter when you’re that far out of school is not that bad unless the school explicitly insists on you having two academic letters. Ask some of the professors where you did well in their classes but don’t have a writing sample, give them something from another class that you did that you’re proud of, give them a transcript and resume, and thank them. See what happens. But in the end, you should be fine with one academic and one personal letter.
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I’m surprised this seems to be the prevailing opinion, since I didn’t get the sense from his email that he’d be unhappy to do it or that he’d write something less than mediocre 
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When a prof says “you should ask someone else,” you probably should. It sounds to me like the other prof options you have aren’t going to write anything worse.
Here’s the thing; it’s absurdly easy to get a glowing recommendation. Why would you risk it on a lukewarm response? There has to be SOME professor out there more enthusiastic than this person.
I’m really, really not confident in this. There certainly could be, but am I really willing to risk just not having a second letter at all?
Now I’m really freaking out.
Chill. Ask before you start freaking out. Tell the short-letter prof you need some time to decide. Ask 3-4 other people simultaneously. Ask your work, too. You will be fine.
Why don’t you just ask all of the people you are considering, and then once submitted, you can choose the letters for each school? If nobody else submits, use this guy’s if you are so worried.
I think at most schools it’s sub-optimal but acceptable to have one academic and one professional LoR. Yale’s Dean did say only having one is a major mark against you for her school, and that might be true at the other top ones. So I’d keep looking, but if you need to submit with only one academic letter, it’s probably okay.
Also: don’t tell someone it’s “insanely easy” to get an amazing LoR when it’s obviously not for some people. Lots of people go to big schools with few small classes, lots of people are shy or whatever and don’t participate, etc etc. This isn’t that unusual of a problem and it’s pretty unhelpful to just dismiss it.
Ok, I’ve now got 4 requests out pending response. I’ll let the first prof know I need some time. I think that’s the best option