You can use the one thing lead to another trick. Ask him over to your house. Play a few games on your computer, log onto a porn site and start masterbating. He'll probably follow suit. Although, this could blow up in your face and he'll be disgusted and leave.
Alternatively you could just be chatting at your house and start talking about growing up, then puberty, then penis size and ask how big is his. If he tells you, just say, can i have a look? Chances are he's curious as well and might want to see yours.