I'm wondering what you've seen that makes you consider him as loose/passive in only 10 hands. What have his actions been prior to this? Has he taken anything to showdown? etc, etc. Which leads to: What sort of PF calling range would you put him on and are you ahead or behind that range?

I'm not sure that I like the 3bet pre OOP in this case, facing a raise & call. With no reads on the btn, you could be up against a decent hand. I'd just call pre.

To answer your question OTF, you can c-bet smaller to get away cheaply, about 1/3 pot, but the flip side of that is that you offer him better odds to continue. On top of that, you call him loose/passive which implies that he's going to continue unless it's really obvious that he needs to fold and even a 3/4PSB prolly won't do that.

If the btn is truly loose/passive then the whole hand is played like crap because you don't want to bluff these guys and you don't have a hand until the river.