Has he opened an eBay/Paypal dispute? If so, just follow the procedure. When asked for your response, offer full refund and send postage label.
He has x days (30 I think) to respond or return, then the case closes.
So the outcome will be binary. Either he’ll return it within the timescale and Paypal will refund him, or he won’t. Whatever he does with the item in the meantime doesn’t really matter to you.
If the manufacturer is as reasonable as you say, you can explain all this to them at a later date and it sounds like they’ll be cool.
eBay is full of utter spunktrumpets. Just let the system work the dispute through, and don’t worry about whatever other dumb shit the buyer wants to do.