Como dizer "referente a" em inglês

How can I say "referente a / referente a que" in a conversation?


Yesterday I was chatting to my father and he said "acho que ele esta certo num certo ponto" then I said "referente a que?" then I was wondering how I would say this in english if I were speaking in english with him.

I also would like to know how I could say "referente a" something that we always use in portuguese


Thanks