Just upload the earliest recording to the web, I'm sure their are plenty of "time stamps" etc. their that could prove prior art
Truthfully, the only thing of significant value is the "lyrics". That is the publishing, or "where the money is at". It is unfair, for sure, but unless he owns the words I wouldn't bother.
And if he doesn't own the words, for the love of all things holy, push him to get a hold of them somehow.