() a 'working vacation' isn't really a vacation

() make sure there aren't other personal matters which make his indefinite presence in San Diego more important -- an ill family member, crucial relationship to be maintained, etc.

() is he still being highly productive, even though not at the level of personal interaction you desire? If so, it's at least plausible that he thinks he's doing his role properly, until he's clued-in otherwise.

() physical presence always helps but it's now very possible and even common for key contributors to be remote, and indeed sometimes that better fits a person's most productive workstyle. So don't get hung up on location if everything else is OK (or can be made OK).

() speak directly with the 'CTO' about your desire for more interaction/mentoring before going to the 'CEO'.

() There are many ways to be 'present' besides physically; give him a chance to deliver the closer involvement/mentoring you think that you and business need those other ways, too -- don't make physically "being there" the only litmus test of commitment.

You concern is legitimate but focus solely on productivity (his, yours, and team's), not symbolism, and express your concern in a way that gives him many ways to address it without making it an ultimatum about returning by a certain date or otherwise making him defensive.