Getting to 20 years offers many advantages. But, disability processing does trump length of service retirement.
I am not sure about your goals...you do realize that an unfit finding with 20yrs or more will basically lock in a 50% payment minimum (same as length of service), and he could get a higher rating? It may be that a fit finding is best for him, it just sounds, from your first post, that you may not be tracking on what a disability finding means. It is the case, though, that getting the 20 years of service will open up Concurrent Retirement and Disability Pay (CRDP).
I think he should be taking the maximum time to respond to every step, and exercise all of his appeal rights (including VA rating decision reconsideration). Given how close he is to 20 years, taking into account processing of his outcome and outprocessing, he likely has a good chance to get to 20 years.