Its going to be a case of one of you going first. When you change the email it will send a link to the current registered email address prompting you to accept and change the it to the new email.
It is entirely possible he will wait for you to accept the email change on your side, then he will simply deny it on his side and recall the account. This is why getting a middleman is very beneficial. To be honest though, he doesn't sound like he's scamming as he is requesting you both do it, but as I said one of you will have to make the first move and confirm the email change.
If you are the OO of the account, it might be worth you going first as you can always contact blizzard and recall if he backs out of confirming your email and tries a scam.
Good luck on the trade.