I was typing quite a long post in reply to this but it all got a little complicated.
I don't believe in ghosts, never have, but weird things happen after people die.
Add kids into the equation and things get weirder. Not necessarily scary, just strange.
Or if you are rational, adult enough and not scared of the dark, you can say that is a sensible explanation for everything. The problem though is that there isn;t a rational explanation for everything in the world, and kids might just see things that adults don't. Both of mine have.
If it was just a one off, your son was probably just dreaming. If yours is anything like my experiences – both my kids would see and talk to my dead mother even the one who never met her " There's a lady at the top of the stairs, Daddy" "Oh great! DOGS COME HERE!" – then it will stop as quickly as it started.
Now who's for a night ride?