Is it possible to be admitted to CS PhD programs (in the US) with scholarship at age 18? I have a son (currently 16) who is a junior in computer science at a well-known university. We are Canadian. He is very passionate about AI research and has published several first-authored papers in highly ranked journals and conferences (top 1-5% in Scopus).

Given the competition in graduate applications in AI, does he stand a chance to be funded in CS Ph.D. programs at 18 (I cannot afford his graduate study)? Will his age be a problem to be admitted to top programs? Should he apply for Master programs first? He has a perfect GPA and GRE.
[For context: I am a professor of mathematics at the University of Georgia.  I've served on my department's Graduate Committee, which does graduate admissions, for about eight years.  From 2016-2019 I was the Graduate Coordinator, hence the faculty member most directly involved in the graduate admissions process.]

Regarding your son, you told us: 

1) He is a junior studying CS at well-known university.  He has perfect grades and GRE scores, and he has multiple first authored papers in highly ranked journals and conferences.

2) He is 16.

To my mind, the first point is highly relevant.  In mathematics a student with that profile would probably get into some of the top PhD programs in the country.  I am confident that the same holds in CS. Moreover, in both of these fields, admission to a top program comes with full funding the vast majority (over 95%) of the time.

I am also assuming that since your son is 16 now and in his junior year, he will be 18 -- i.e., an adult -- when he starts graduate school.  Given this, the second point does not seem relevant to me.  Some people will find your son's achievements (even) more impressive given his age.  Others will have some concerns about the maturity and readiness of such a young student for a PhD program.  But when it comes to admissions and hiring, times have changed: in the last few years faculty have received much more training and specific instructions (including on legal obligations) regarding admissions and hiring than in years past.  Deciding not to admit a qualified student because of his age sounds like a discriminatory practice to be avoided.  Conversely, giving more weight to a student because of his age sounds like a discriminatory practice against the other applicants...also to be avoided.  A small number of committees may still be influenced by such thinking, but overall it should be the case that your son will be considered only on point 1) and I think that it will be the case.   

In summary: your son has an excellent profile that will most likely lead to multiple fully funded offers at prestigious CS programs in the US.  (By the way: yes, he should apply directly to a PhD program.)  You do not need to, nor do I think you should, do anything out of the ordinary because of his age.