Charging Rent?

Today DH's best friend is defending his thesis. Then he's moving up near us and is planning on staying with us. I'm not sure how long. I've cleaned out the guest bedroom, had the cable turned on, and on Friday a friend of his driving up dropped off about 15 boxes of stuff. We moved that into the guest room and it's starting to look really full.

But I was thinking, should we charge him rent after like 3-4 weeks? Is it appropriate? Or do we just let him pay for food? Do we charge him for food after 3-4 weeks? He's stayed with us at most 2 weeks at a time, which has never been a big deal. But previously those were visits, not actually living here.

I think he'd be amenable if he can't find a place to live, but is it rude? And should we even bother? And how much do you even charge in this case?

Also it's possible he'll find a place soon after moving in. He's not really the type to be on top of things, so I worry he'll move in with the intention of moving out, but gets lazy and doesn't. And I like him, so I can't just kick him out (no way DH's kicking him out). But I think I can deal with a month.