Another concern you should consider...
If you do commission another bass from him this would constitute another contract and could nullify your County Court action against him.
As others have said, there is no guarantee of the quality of the new build. Unlikely that you'll be a returning customer so what's his motivation to do a good job?
I'm with the majority here, get your money back and get in contact with @Andyjr1515 to build you what you want.
There's a reason why he's always busy.