It depends on what you call "artwork". In the French law, "original artworks" are not subject to VAT, but the artwork must respect a list of criterias to be considered "original".
I highly doubt that reproductions printed on a canvas are "artworks" according to the French taxes code.
As a result, when you ship the prints back to him, your client WILL BE CHARGERD VAT (19,6% rate) by the French customs, even if you are actually reproducing his own original artwork. I don't think that you, being the exporter, have anything to worry about, except producing an invoice which indicates the total amount (without French VAT) you charged your customer. This will be the amount the French customs will use to determine how much VAT must be charged.