[LAFwUs]

I've had the same thing happen on TWO vehicles,
a mustang frame off resto & vintage gen 1 bronco. Two different shops.
Painters/body work guys are never, ever done within their stated completion date. Him stating 2-3 months was fantasy. He maybe just having labor / supply shortages like everyone else, or he's sandbagging you. Only you can determine that.

Before you head down the path of retaining an attorney, some things you may need to consider.

If you do, he can & probably will, place a mechanics lien on the car for services rendered/payment due, for work completed thus far. He can do that without your knowledge, even if he releases the car to you.

Understand that if you request the current invoice, so you can take the car elsewhere, that bill will be well padded. If you choose not to pay the padded invoice, you will not get your car back and its perfectly legal.
He can also, at that point, begin charging you 'storage' fees, under the guise of the car is no longer his problem (contract to complete terminated), since he is no longer employed to work on it. This can lead to other issues (missing parts, neglect, damage, etc)
He can also make it nearly impossible for you to actually then retrieve the car. (never available, parking it in the back of the shop buried behind other vehicles, not putting the car back together enough to be a roller, relocating it to "his other shop", etc)
I'm not saying, to not get an attorney, but it may only complicate things at this point, if he thinks you are going down the legal rabbit hole. He actually has the upper hand to make your life really hard, leaving you to deal with the aftermath, so to speak.
Consider a softer approach.
You could attempt to get the car back, by saying it has to go to another shop for other pre-slatted work (like upholstery, or glass, whatever) or simply state, that 'your out of money' and need to fold back for a bit before the wife divorces you, but will be back, just in order for him to release the car to you without hostilities or overcharges.
This worked on my mustang fiasco, although it took some tip toeing and patience.

The BBB is utterly powerless. They have zero enforcement authority, other than strongly worded letters and a complaint report posted on their website. Few businesses, especially shady ones with an already bad rep & unhappy customer base, actually care about the BBB or their ratings.
Now: facebook, yelp and car club/forums however can have great affect in warning others of a shops bad behavior, if things end up there. That bad rep could be unrecoverable for a small business, something I would make him well aware of - again, if things go that far sideways.
No matter what though, document everything and try and stay on the legal side of things - in case it does go to court.
I understand all too well the frustration.