Tricky, but you could take the approach of just putting it on the table:
"Hey, I have a weird random question… " And then ask if he thinks there is life elsewhere in the universe or what he thinks is going on with flat earthers or whatever. It isn’t bad to be a bit random in timing.
In terms of timing specifics though, you don’t want to do it in the middle of some other discussion, so you normally wait for a quiet moment or a lull in the current conversation. Typically depends on what’s going on. I find something like building puzzles together is great for this, because you’ve both got something to do but it’s not so intense as to block easy conversation that just meanders a bit. Just an example though.
When you are together, what are you normally doing? Long drives are easier to find ok moments, studying in the library or in the middle of a dentist appointment less so