The answer to your questions should have set out in your agreements and contracts but sounds like this was a little more casual. Therefore, your past royalty distribution pattern should be a good indicator. If that does not satisfy the band member I would suggest scheduling a short mediation and have an independent 3rd party help you sort out the mess and draft a new agreement that will serve you well in the future.
Yes, generally a leaving band member gets their fair (equal to the other band members who played on the recording) share of the artists' royalty on sales of the recording, less his pro rata or other agreed share of any expenses related to such royalties (e.g. recoupment of advances, legal fees, costs of collection, etc.).
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
If the former band member performed on the album he is entitled not only to his pro rata share of all the profits earned from the album sales, he also owns an undivided interest in the copyright in the sound recording rights to the album. Which means that he can distribute the album (either digitally or in hard copy), license the rights to others to distribute the album, and perform the songs on the album -- all without asking you or anyone else for permission. He would, of course, have to compensate the songwriter (through BMI, ASCAP, or SESAC) for the statutory musical composition compulsory license but that is an automatic process.
Your question of "what is fair compensation?" is irrelevant. The law IMPOSES the compensation to which he is entitled: his pro rata share of profits and the grant to him of a non-exclusive right to exploit the sound recording copyrights.
All of which demonstrates why not having a band partnership agreement is simply .... not wise.