If the board is secure and basically in-plane, the only real consequence of sloppy hanging is more work for the finisher. So, unless you believe he's going to do a subpar job at finishing, then there is no problem yet. What level of finish did you agree on?
If you don't believe he will do a quality finish, then about your only recourse is to negotiate a break in the contract and wish him good luck and hire someone else to finish.
I would advise that you have a polite conversation with whoever shows up to do the finish work as to your expectations for the finished product as agreed to in the contract.
Ask the guy how he is going to fill the gaps, and any other concerns you have and listen to his answers.
You always have the right to end a working relationship with a contractor, but you don't have the right to get your money back or "win" financially. AKA: You can "fire" him, but you still owe him money. Ending a job midstream always includes some grey areas for a financial split: scheduling, materials, what percentage complete etc.
Right now, I don't see anything in the photos that precludes them from delivering a quality finished product, or give you grounds for breach of contract.
That said: if you are convinced that they aren't going to finish the job properly, you should absolutely cut ties and negotiate an end to the contract.
FYI: Here is a possible explanation from his end. (aside from being rude to you, which is just dumb) It's a big company as you say. they make their money at scale, doing whole houses/tracts of houses. This job is not really profitable for them and doesn't really fit in their business model. But takes the job anyway. Sends out the expendable apprentice to hang the job. Apprentice does a predictable job, but that doesn't concern him because he knows the experienced guy he sends out to finish it can fix anything the kid screwed up. Completely fails to consider the effect on morale on the customer from the shitty hanging job because he doesn't normally deal with customers in this situation.