If he’s anything like I was when I was a skinny guy, I think I can give some advice. 

If you’re going to be ordering a pizza, I imagine his plan is to stuff himself with it to sprout a bloated little belly.  You’ll win a lot of points if you try to encourage him to eat past his limit and maybe rub it afterwards.

If you get to fool around, give it some attention if you’re into it.  If you are, talk about how you’d like to see it bigger.  That will really push a lot of buttons.

Assuming he’s a skinny guy with a full belly, it’s not going to be anything too big or intimidating, so who knows—maybe you will really want to see it grow bigger.  I think this is a really good scenario.  It’ll give you a good chance to see if this is something you really like.  Good luck!