the vacation excuse is overly used on ebay imo.
he should ship you the correct fuge along with an extra label in the box what has his addy as the ship to. then you can put the unuseable one in the box, slap the other label on and ship it off. thats what i do in those situations and i dont even question them or waste any time. either that or i just let the buyer keep the wrong item. depends how expesive the item is.
you are definitely not being unreasonable asking for him to pay for his mistake. thats a sellers job. if he doesnt want to do that, i'd leave him a negative and start the process of getting your money back. you can try to mess around with a paypal dispute but i'd just go to the credit card company and do a chargeback. this is also why i always use my amex card when ordering online. they rarely side with the seller. anyways, if hes even considering the idea of you haveing to pay to ship it back to him, he deserves the neg. just my opinion.
also, pay attention to how much time is passing, dont allow him to stall because it is possible to wait too long to get your $$ back. this is difficult because this is a custom item. remember how long it took from him to build it? if it takes any longer than that, i'd get on his butt asking where the item is. i dont have all the numbers in my head but there are limits with paypal as to how much time can pass before you can dispute a transaction, before you can leave negative feedback, even some CC companies will have limits on how much time can pass before you can do a chargeback. be aware of these deadlines and act accordingly. its a game you gotsta learn to play it right!