Hi,not an easy answer.
If you are undetectable it means you are untransmissable U=U and you should feel confident when you approach him. You might carry some internalized stigma and your partner can sense it. You have done nothing wrong but also seems that your partner needs some HIV and STIs education. He wouldn't have any problem kissing you or any other potential partner if he was unaware of yours or their status...
A way forward would be to offer him the opportunity to HIV educate himself by taking him to meet your doctors and I'm sure they will be able to answer any questions he might have.
If after that he still feels that he can't be romantic and you are not ok with that probably he is not the right choice for you, despite your emotional investment.
Look after yourself.

He has to be all the way ok with it or not at all. Not kissing you is saying he is afraid. Let him take his scared behind elsewhere. Because no matter our status we still deserve unconditional love. I prefer to date someone with the same status if possible. That way that's one thing that wouldn't hold us back from having the love we want.

If you are undeniable you really have very, very little chance of infecting him. Kissing can not pass the virus.

I'm undetectable. I even regret telling him.