I found my dad's adoptive family but he doesn't know - should I tell him??

Hello, I've been researching my family tree recently and have discovered lots about my dad's adoptive family and even located some of them on Facebook. However he has never shown interest in looking for them, and I'm not sure I should tell him. On the one hand he has a right to know surely, but on the other he seemingly doesn't want to know and might even be angry I've 'gone behind his back' and researched all this. Should I just keep it to myself?