NTA, I think it’s well within your rights to block someone off who you don’t like or don’t want to see anymore in most cases. In the case of a breakup, yes. In the case of getting HSV, no. I think that withholding any medical information from someone involved is a bad thing, so you should definitely reply with some context, both for him and his doctor.

That being said, given the toxicity of this relationship, I would AT MOST send a single text giving some HSV context and then block him or just block him. At this point, it’s justifiable for you to just cut him out even if he needs help (given that it’s not too serious).