When YOU use paypal, you will create a paypal invoice with his contact info and they will generate an invoice to send to him.
Paypal will send an e mail to notify you when the money is in your account. Then it is safe to proceed with the shipment.
It is well worth spending time ith paypal behind their scenes to familarize yourself with their process, fees, etc.
Before this, however, you need to make sure you know what painting he wants, how much he has agreed to pay for it and your shipping costs...whether added on to the price or you are including it in your price. decide what shipping method works best for you, check online to see how much it will cost, how long it takes to get to the destination, etc.
What I am saying is that the invoice is the LAST thing you send him to be paid before shipping the painting out.