"Artists Beware"

I agree with some of the others above of give him a deadline to get back to you about the expressions or you'll default to three of your choosing, maybe even say which ones so they are aware. Or make a version without the expressions and keep the file to add them at a later date whenever they're ready. If they're otherwise pleasant to work with I don't think you need to refund them and cancel the project. Let them know if they don't respond you'll assume they're okay with your decision on the matter.