He helped me get 1 black book in dwemer ruins and after the quest was done, I swear he said he would tell me the location of more black books, but when I talk to him he has no options about other black books.

Am I doing something wrong?

finnish the Dragonborn main quest then talk to him in Tel Mithryn.

I have characters that already had all of the Black Books, and Neloth will not have the Topic appear in Dialogue.

Since this is Cheats, Hints, and Spoilers- There is a Mad Man running around Solsthiem that spawns a Note that can add a Quest to retrieve a Black Book, but for a character that has all of the Black Books, the Note will give a Journal Entry that can never be removed without using Console Commands.

Neloth will give a Verbal Hint when He has the ability to send Our character after an undiscovered Black Book, just pay attention to His Dialogue.

you have to complete the Dragonborn questline first