Ma'am,
There's at least one more appeal step from here (called a Petition for Relief or a Rebuttal, depending on the service branch). So, all hope isn't lost yet.
I certainly understand your frustration about the FIT findings, especially after your husband's treating physician gave an opinion that he's not okay to go back to work. As of January 2018, 6% of all Service members going through the DES process were found FIT and returned to duty. But that varies dramatically by service branch (2% for Army, a whopping 30% for Navy). It's a tough pill to swallow, for sure.
Something for your attorney to consider when filing your appeal - the Board is not supposed to substitute their own judgment for that of your husband's treating providers.
One of the fundamental guiding principles of disability evaluation is that the Board is not permitted to rely on its own unsubstantiated medical opinion in disregarding a treating physician’s diagnosis or findings.
The seminal case on this point is Allday v. Brown, 7 Vet. App. 517 (Vet. App. 1995). Allday states, “Moreover, the Board must support its medical conclusions on the basis of independent medical evidence in the record or through adequate quotation from recognized treatises; it may not rely on its own unsubstantiated medical judgment to reject expert medical evidence in the record, but may reject a claimant's medical evidence only on the basis of other such independent medical evidence.” A quick Lexis search shows this headnote cited by 4,840 other cases, including 1,988 times since 2010. It is very well-settled law.
A recent case clarifying the issue is Reyes v. McDonald, discussing the requirement that the Board provide thorough rationale for disagreeing with medical opinions in the record. “To comply with this requirement, the Board must analyze the credibility and probative value of the evidence, account for the evidence that it finds persuasive or unpersuasive, and provide the reasons for its rejection of any material evidence favorable to the claimant.” Reyes v. McDonald, 2015 U.S. App. Vet. Claims LEXIS 978, 8-9 (U.S. App. Vet. Cl. July 17, 2015).
There is a ton of other law out there on this point, because it's something the Board and the VA get wrong all the time. My guess, without reading your husband's Formal Board rationale, is that the Board didn't do a great job explaining why they disagreed with the treating physicians. I feel a little like a daytime prescription drug commercial, but "ask your lawyer if this argument is right for you."
-Matt