Who is a Co-Author?
Lets say that the other members registered the songs and did not list your husband as songwriter. If your husband challenges the registrations, a court will be concerned primarily with whether the percussion parts and arrangements actually amounted to a contribution that merits co-author status. According to the Dear Rich staff, courts have not always looked favorably on percussion parts and arrangements as compositional contributions particularly when they are "dictated solely by musical convention or tradition." (Cf. In this case, an arrangement by Billy Strayhorn merited copyright protection). If your husband is convinced that he is a co-author, he can file a copyright application listing the appropriate information. (Here's a video we made that explains the process) But keep in mind that filing a copyright application with false information -- no matter who does it -- may lead to a claim of fraud on the Copyright Office.
What's the Deal?
If the band members agreed that your husband deserves a 20% portion of the revenue from those songs, he is entitled to it, regardless of the claim of authorship. But your husband will need to prove that an enforceable agreement was reached among the parties. The verbal agreement may suffice though it is will be difficult to prove and may prove difficult to enforce for other reasons. Written documentation of the agreement (including emails) would be best but its hard to tell what proof is proper unless an attorney reviews the documentation. (PS we recognize that this is not a laughing matter, but a segment of our staff insists on linking to drummer jokes!)