Paypal is good some times.
In this case, you should be covered, but yeah, just escalate as soon as you can.
You created the dispute, I believe there is like a 7 day or 10 day grace period before you can escalate.
As long as you're 100% in the right and he hasn't got proof of postage, you'll get your money back.
Whatever you do though, make sure its all good before you escalate, as if he shows proof and somehow gets the dispute closed (no money to you). You wont be able to open a dispute again and if he didn't post, you get nothing again.