It sounds to me as though this is something of a breach of contract, but it can be hard to pursue with just an employment letter and not an actual contract.
What pops into my mind is that you may want to actually hire an attorney who specializes in employment, and particularly in contract disputes, and see if a solution can be mediated. Sometimes the best solution is for the parties to sit around a table and negotiate. It sounds to me as though the employer is really taking advantage of your husband. It's also a good-paying job, and it sounds to me as though your husband would probably want to keep it if it were not for all the BS, so filing a wage claim or a legal action may not be the way to go here.
You can see an employment attorney for just an hour consultation and get some good ideas. I'd look for someone with a minimum of five years of employment law experience. Don't actually HIRE an attorney until after checking references. The attorney can review the letter and the facts and give you advice. Unfortunately, I can't give you advice, just information.
Here is one place to look for such attorneys: http://welaweb.org/
Be cautious, however. Any attorney can join this organization; it doesn't necessarily mean they're experienced or good.
Most WA counties also give referrals through their county bar offices (I don't know what county you're in). Usually, you pay something like $30 for a half hour consultation. Again, these attorneys aren't necessarily good - in fact, some of them may be new attorneys trying to get more business.
You may want to try some of the larger law firms, with more resources. You may pay a little bit more, but you get what you pay for.
Keep in mind that if and when you hire someone, you need to know whether your retainer is refundable or not. If the attorney doesn't want you to sign a representation agreement before you start, leave.
I wish I could recommend specific attorneys, but I'm not allowed to do so here.
Expect to pay $200 - $300 hour; some of that money will pay for research, too, so it can be expensive.
Here's another idea. There are some non-attorneys who are excellent at negotiating/consulting, etc, and they charge less (should you decide to try the negotiation idea I suggested). Some of these folks may be former union/labor people. There are probably such people in Olympia, Seattle, and Spokane. (I knew someone in Olympia who was excellent, but he may have retired by now). Again, check references.
Now, your husband can file a workers' rights complaint and see what happens: http://www.lni.wa.gov/FormPub/Detail.asp?DocID=2367
In theory, he'd be protected from retaliation for doing so. In real life, well, retaliation can be subtle.
Meanwhile, make sure your husband keeps excellent records of all costs and overtime associated with this position.
It sounds to me as though the company is a slavedriver and thinks it can get away with anything. But it also sounds like a breach of contract - it's just hard to prove when there's no actual contract.
I hope that that helps. Since it's Saturday, I'm going to take a bit of a break, and will check back in an hour or so to see if you have any follow-up questions. I'm on the west side of Pierce County, and it's finally not raining, so I want to get outside and do a tad bit of yardwork. If you're in Western WA, you can probably relate, lol! I hope that you don't mind.
Jane (in case I forgot to mention this, I'm a licensed WA atty, but use a pseudonym while working for Just Answer).