I had a friend I had known for a bit ask me "hey can I ask you about your stutter?" and asked me a question. I agree that it's nice when people don't approach it like it's some taboo. However, for OP I would suggest to wait at least a little bit before asking. Doesn't have to be too far but when you can tell y'all both feel comfortable. If I was him I wouldn't mind being asked on the first date but he might still feel bad about it.