Easy he's your best friend which means you sure a lot with him, why not start by showing him some pictures of your wife? Maybe start slow with some pictures of her in a sexy dress and heels then move on to more revealing pictures of her body.
I'm sure you and him talk about fucking other girls, you can try talking about how your wife likes to fuck, what position she likes, if she likes to suck and swallow or maybe anal.
There's plenty of ways to bring up the subject of wanting to share her, if she's your best friend I'm sure he'll be interested.