When he says that he wants to take it slow, what does that mean?

Have you asked him to specifically define "taking it slow"? You can ask him without it sounding like an interrogation.

You have the right to ask about stuff like this, because the relationship affects both of you. and if he wants or needs something specific, you can't offer it unless you know what it is.

so talk to him.

But I think there's a huge difference between "taking it slow but deliberately" and "drifting along without a plan." The first is good, the second is a recipe for disaster. So if he can't define specifically what "taking it slow" means to him, it might be a red flag that he's stringing you along or some other bad thing...

good luck.

Guys don't normally want to slow down the physical progress of a relationship, but they are wary of early attempts to nest and tame them. I.e. girl wants to move in together.

But if you're not there yet, it sounds like he could be losing interest.