How did he bring up this statement? Did you ask or did he "announce" it on his own?

Depending on the context, his intention may come off differently...
A) he announced to imply he's "in demand".
B) you asked so he simply just answered
C) the conversation just flowed naturally, he thought he'd tell you as an honest person

Well, you already crossed him off your list, so he's just agreeing to stay friends. Rejection from a side easily turns mutual because it's a total turnoff to a neutral impression.

"Serious" to me was reflection of how he treated the relationship, I don't think that was a bad thing. I guess you were thrown off by the 180° vibe change -- I don't think he meant to deceive or anything malicious... Tbh it really just sounded like maybe he wanted to show a prim and proper self or he was just really socially awkward.

I don't think any precaution is needed because like was nihility said, you can do all your prep work to anticipate, and the reality will still surprise you every time, for better or worse. I do think maybe you can shift your mindset a little: be more open/accepting to one's awkwardness, as long as the honest intentions are there. Give people time to get over their own awkwardness.

1. You say explain, but did you give him the chance to? I don't know how your conversation after the first date went, but if you rejected him before he replied anything at all, there wasn't any chance then.
2. Not everyone is open to share their weaknesses. Maybe he never thought of telling you that because he feared judgement/"deducted points".
3. You are an option. What did you expect from a first date from dating app?

Continuing point 3 above, there's some entitlement from your end. You can reject him but he cannot even "agree to be rejected"? What have you contributed to make him not to give up on you easily? You're not in a relationship yet to "demand" anything.

Even in a relationship, I think it's a constant learning curve to read your partner's mind. I used to hate mind reading but I realised it's so fun to successfully guess what he's thinking or when we clicked on something without saying a word.

I'll give a radical suggestion, it'll be way out of your comfort zone but maybe that may spur a change

Message him back and apologise for jumping the gun, explain that you confided in someone who pointed out the possibility that he was shy/nervous/awkward, hence the change in vibes. (And ask if he really was?)

Fat chance you ain't getting anything back and worst case, you get ghosted/blocked. But, I think there's a slim chance, just maybe, that he was slightly interested so might agree to a 2nd round and maybe with this honest exchange, something may feel different. Hear each other out about your feelings without judgement, maybe with this honest exchange you might learn something new. If you want to increase your chance then treat him on 2nd round lo.

Caveat: only do this if your gut feeling determined he's a sane/normal guy. Someone crazy might take revenge/humiliate you.

Why do this? Well, mainly is to humour me lol kind of an experiment to see how things will go. Personally I did things way out of character and learned from the outcomes and also my own feelings what I dis/like, and there I was able to change for the better (or at least what it means to be me version that I'd like).

Lol that's a nice start! Please keep your hopes in check ya haha you sound very excited 😂 give the awkward guy time to warm up, good luck!