Oh forgive me, I wasn't very clear. This is an existing boss.
I have read countless articles saying that it's illegal to ask during an interview. But, what about when a current boss who's asking this? I mean, it wasn't even a casual conversation. It was at the end of a work performance review. I was bothered by it and was taken aback, but wasn't able to react fast with better answers that you guys have. I was only able to come up with, "yea, maybe later" and shrugged it off and focused back on work related topics.