I think if he wants to meet his Dad then you just say "sure, when you're old enough to go visit, I'll help you find out where he lives". My younger sister has a different Dad to me and she always knew. She's never asked to meet him though. Sometimes she asked a few more details and I'd tell her whatever I knew of him and her other 1/2 siblings but that's as far as it's gone. My dad has told her he thinks of her as another daughter and I think she's happy with that- she sees him as her father figure.