he is mostly into things that earn him future favors and influence, BUT i was just typing at china that jack may be tangentially dabbling in providing information to assist smugglers who are bringing irkallan refugees into the city, which seems relevant... so if lexa is familiar with william's beau and vis-a-vis william, they could have a connection there.

jack is generally (although not publicly) in favor of making peace with and/or reuniting as much of the human population as possible and then consolidating power under himself maybe, what, so he may be a useful ally in the future, too.