Are these big-ticket items we're talking about, are they mostly smaller/lower-value items, or is it a mix?
It might be helpful to probe a little and find out why he doesn't want to put prices on anything. If he's afraid that his prices will be too high and his sales will suffer, you can point out that sales are already
Personally, if I walk in to a store and see merchandise on display, I expect to be able to pay for that merchandise on the spot and either take it with me or make arrangements to have it delivered (depending on the size and nature of said merchandise). There are times when I may come back a couple of times to look something over and consider whether to buy (especially for big ticket items), but in general -- having made the decision -- I want to be able to pay and go. I would not be happy to learn that I then have to wait for a clerk to phone the business owner, learn the asking price and call me back before we can even begin the negotiation process.
He doesn't have to post prices on the pieces if that isn't standard practice among your competitors, but he does need to price the goods and furnish you with a price list. I think most customers in antique shops expect to be able to negotiate on the price, so he also needs to empower you to negotiate (perhaps give you an initial asking price -- maybe you'll be lucky and the customer will just pay that right off -- and also give you the "rock bottom" price below which he will not go, with your instructions to bring the sale in at the highest possible point between those two extremes).
If he's not willing to give you a price list and he's not willing to allow you to negotiate on the store's behalf, his only other option is that he needs to be on-site all day, every day while the place is open so he can negotiate directly with the customers when they're ready to buy. Otherwise, I think he's going to find that his sales continue to languish.
This will likely be hard for him. Business founders are notorious control-freaks. He probably believes that no one else can negotiate as effectively as he can, or that no one else cares as much about the success of his business as he does. Unfortunately, he's going to have to loosen his grip or he's going to lose the store.