It's nice to hear you're interested in medicine for your son, I hope he is interested as well.
UMKC's 6 year BA/MD is one of many BA/BS/MD programs in the country. Simply google BA/MD and many things will come up.
I have applied to 11 programs this fall, and am hoping to hear back! Here is the link I used: Medical School Programs
Many of the questions you asked can be found by doing a bit of digging on the 6 year BA/MD site. Here is the link: The B.A./M.D. Program : Admissions : UMKC School of Medicine
Participating in UMKC's 6 year BA/MD program has been my dream since the 6th grade, so I know quite a bit about it. Being an out of state student does matter. Missouri residents get priority, then regional students, then out of state. Out of their nearly 112 spots, 60% go to Missouri residents, 25-30% go to regional, and 10-15% go to out of state applicants. Overall, in the past couple of years UMKC has only accepted 15-20 out of state applicants, out of the nearly 1000+ total applicants. So yes, your chances improve significantly if you live in Missouri. However this program is not reason enough to pack up your life and move. Even if you do move there is no guarantee that your son will be admitted. While I'm sure his stats are outstanding, there are many other factors such as the essay and interview that go into determining admission.
I would take a look at other programs in the nation before you make any hasty decisions.
I hope this helped, and best of luck to you and your son!