My take is that if he was hunting for sex on Craigslist & wanting bred, and didn't ask your status beforehand, then it's totally his responsibility. Don't feel bad, no reason for you to! People have to take responsibility for themselves. If he was looking to be bred, then he should have assumed there would be a risk. He knew the risks, and if he didn't then he's just dumb. He should get educated about it, as you say, being undetectable is an extremely low risk of transmission anyway, as I often explain to guys. often wonder about this too. Like on Grindr, where it's not always assumed by guys that you might be poz, unlike say BBRTS or a poz-friendly site. Since you're undetectable, like me, there's no way that you could have given him "fuck flu" if that's what he calls it. The guy has some issues clearly. He's probably slept with a ton of other guys & is just paranoid.
The reality is that many guys still are ignorant about hiv. Being undetectable and healthy is a much lower risk of transmission than a high viral load. Personally, I hate when a guy asks if I'm "clean". If a potential hookup asks that during chat, I stop chatting with him, because it's rude. You can ask, are you hiv poz, and I'm happy to discuss it, but being hiv positive does not make you "unclean". Also, hiv is a chronic, manageable condition now, not some deadly disease, with treatment. It's not the 1980s anymore. Guys like that really need to get with the times & learn what undetectable is. Being poz today means you can live a long full healthy life with treatment.
Here's where I come down with regards to hooking up as a total top poz man. If the guy point blank asks me, like on grindr, I tell them, I'm not going to lie. If they're open, great, if not, oh well, I wouldn't want to breed someone that shallow & uninformed anyway, & at least I can sleep well at night, & besides, plenty more fish in the sea. If they don't ask at all, then I don't feel obligated to tell them or bring it up. Although, on sites like bbrts which is poz-friendly anyway, or a4a, I state my status as undetetcable. But on Grindr, I don't tell unless they ask me directly. That approach has served me well.