No such thing as non refundable if they failed on their end. Would never hold up even in a small claims case. You would win in a court even though a contract said non refundable as they didn't hold up their part of the agreement. You would get your full money back.
Tough part is they are probably a long way to file a case against. You may be able to do it where you live though. You could also contact his local district attorney and file a complaint. If he tries to use an excuse that he has material costs doesn't matter as you can't be expected to finish the cue yourself. he would have to eat it, or sell to someone else.
Give him a deadline of say a month or so, and if not done you're filing charges for theft, then follow through with it.
Some cue makers are the worst, sort of the same as custom pistol smiths. If you were local and at their door every day, they would finish it, but in your case, out of sight, out of mind.
There are very highly respected cuemakers that finish cues as promised, and usually within a year to year in a half.