I don't know the statute number, sorry. This is all recollection from when I was a CA contractor and studied for the test.
That said, your contract includes the very wording I was referencing, so your contractor has two strikes against him: the law and that clause! Nice of him to provide in the contract the very provision he has violated.
The "Additional Spec's" section might be an example of the other point I made: that clause could be interpreted as violating CA law regarding change orders, and so is probably null.
More importantly, IMO, at least the first paragraph of that section doesn't help his case at all. It states what is included in the contract. OK, fine. Then it describes what the charges will be
should extra excavation be required. That's a good provision to include, for sure. But it doesn't give him carte blanche to perform any amount of extra work without your consent/knowledge and then later charge you for it! I'm sure that's what he intended when he added that clause, but it doesn't actually state that. That clause in no way eliminates his responsibility to issue a Change Order for the extra work required! This attempt by the contractor's contract to circumvent the very reason a Change Order is required by law doesn't succeed.
If anything, the second paragraph is more of a problem for you, because of the words "Buyer(s) will be responsible for the difference." But there is another CA law that might come into play here (I think this one pertains to all contracts, not just construction contracts): the creator/provider of the contract (in this case, the contractor) has the burden to include text that is not ambiguous or otherwise faulty or confusing (or illegal) in some way. Should the disputed text prove to be unclear, then the other party (you) gets the benefit of the doubt, and can prevail in the dispute just for that reason alone. And I think also that he would be doubly at fault in that regard, because he is the professional, and the seller of the service, you are the consumer. It's on him to make his own contract unassailable. CA is very pro-consumer in these types of matters. So he has a lot going against him regarding these extra charges.
Speaking of being clear, my experience with these laws is based on my years as a CA Contractor, and the studying I did (1) when I took the contractor's test and (2) when I researched CA Law while going after CA Contractors for various reasons. I'm not a lawyer and didn't use any to win my cases. I studied a bit on the 'net and wrote demand letters to the offenders. I prevailed several times in court, but the highest-dollar case I won never made it that far. I involved the CA Contractors License Board and they somehow convinced my pool contractor to pay for my new pebble finish. Point being, I think this is one of your next steps. Go to the
Board's website and file a complaint. Start that ball rolling, 'cause it takes a while. You'll explain the problem in the filing, and eventually get assigned an agent. That person will call you to get your side of the story, then call the contractor to get his, and hopefully help you two resolve the issue. This is at no cost to either party, so it is a good first step before court and/or lawyers get involved. Also, that agent should be well versed in the laws I've been referencing, and that person would be the resource for which statute(s) apply, and how they apply.
One thing though, there is a bit of chicken/egg going on here. You haven't actually been damaged yet, I think. You have your pool, you paid the contracted price (I presume), the end. It's not until he sues you or liens your property or dings your credit that you actually have something to resolve. So I'm not crystal clear on what you should or could do at this point, legally, or that you have to do anything at all. You should probably respond to his bill with a letter disputing the charges. State either that you don't have to pay (based on the arguments I've given you), or that you feel the charges are unjustified and that you would like to discuss them before proceeding. The latter is the more reasonable course, especially if you feel that you legit owe something, but not as much as he's claiming...
Either way, I would strongly recommend that you do this all in writing. He'll likely hang himself, they usually do, and when he does you'll want that in a letter, not within a phone call, which invariably turns into a he-said-they-said situation (which judges hate). Be polite, be reasonable. Include absolutely no opinion or emotion in the letter(s). Just state facts, or ask questions (both are legally ambiguous, which keeps
you from hanging yourself). Maybe save the legal threats for the second letter.
@PoolGate has made a great suggestion. In the first letter, question the amount and ask for a complete itemization of the charges. Perhaps include a request for invoices from subs, and/or their contracts and change orders, etc. Make him work for it...