@mr_big_t
It is confirmed in these sources that he indeed refunded you your payment to the same address he received it from.
This is where you can find all of the generated address for each wallet
from you to him 14/8/18
https://live.blockcypher.com/btc/tx/f3220a6398484e924861b6278659f51a6cfed8570dd040384a71b65dd3d910e2/
from him back to you 14/8/18
https://live.blockcypher.com/btc/tx/4a6a13ec253e82a27fcce158a7a003a6669ee26447dfb62f9b5987fbc6d00ef2/
From the evidence gathered it is clear you have received a refund. Yes after a transaction a new wallet address is generated, and you can reuse the old ones. Coinbase implemented this security so you couldn't not keep track of 1 address to see whos holding what amount (privacy). This can be proven in Coinbase's FAQ page.
He sent the amount back to you. This could be an error on coinbase's side if you cannot see this transaction. I recommend you speak with support with the given blockcypher links.