I'm sorry to hear that and certainly his behavior was irresponsible and inappropriate.
I don't like the use of the word promiscuous because sounds judgmental and whether someone is hiring or hooking up on Grindr (or a park as folks used to do before internet) is not our business.
I agree with you that you're also responsible because you weren't on PrEP and I guess you didn't use condoms for anal with him. I'm sure he infected others but hopefully before PrEP was around they used condoms and later on they took PrEP.
As told by others already now HIV is highly treatable. He is also a sick person and he needs help, how come he's not undetectable like so many guys living with HIV? Maybe he's not even responsible when it comes to his own health, let alone others who played Russian roulette with him.
Whoever you contact is your word against his word but as an HIV + individual he must be stopped from infecting others. In some states if he says he lies about his status there must be some legal consequences.