LeftyDad - For certain it demonstrates a great amount of interest. No question about it. I would say do your homework ahead of time so you can get your questions answered.
Now, on expectations for this unofficial visit. I have 2 sons, both play(ed) in the Pac12. They both had these types of invitations. Just anecdotal data here, but this is what we experienced:
Older son (was a big recruit with lots of opportunities) - Took 4 unofficial visits, received an offer on only one of those initial/unofficial visits. All offered later plus more. This was in summer of 2003 when timelines were a bit later than now - son was a 2004 HS graduate.
Younger son (not as big of a recruit, but had multiple opportunities too) - Also took 4 unofficial visits including 1 that was a 2-hour flight away (about the same time as your son will). No offers on any of those the first time. What did happen? Tours, meetings with coaches (including HC), campus tours, academic discussions incl. advisors, gauging son's interest, in two cases our son was put in a room with current players/no coaches and encouraged to ask whatever he or we wanted...in one case a football game and another a basketball game. 2 of the 4 offered later, but I am convinced the other 2 would have also if he hadn't taken one of the first 2 offers. This was in Fall/Winter of 2009/2010 - son was a 2011 HS graduate.
Bottom line in our experience, it was more of a mutual 'feeling out' than anything else. No way does that mean your son won't get an offer...many do. Just advising not to get discouraged if he does not.
One legitimate question in my opinion, "Coach, what time frame are you thinking makes sense for you to make any potential offer?"
I asked it and got what I thought were good/honest responses. One coach for younger son's response was something like, "We really like your son and we will recruit him hard over the next 6-9 months, but we won't make offers until next summer." You may also learn things about academic constraints on offers (e.g. Stanford requires going through entire admissions process after Junior year).