Tough question.
The band originally owned the logo, but then the question would be is the extra work done by the ex-band member a "work for hire?" Almost certainly the band members are not "employees," as defined under relevant state laws, of the band; however, an argument could be made that the work done by the ex-band member was done as part of a collaborative work, in which case the work WOULD belong to the band under the "work for hire" doctrine.
You would need to talk with a lawyer experienced in copyright laws to get a more definitive answer; a lawyer would want to see ALL of the relevant evidence. There really isn't a cut-and-dried answer based on the facts as given -- you really need to consider ALL of the facts to figure out what the ownership situation really is.
In this case, however, you might want to simply ignore the ex-band member and wait to see what -- if anything -- he might try and do. If he does something to impact the band, THEN spend the money on a lawyer.