Career scientist here. MD PhD path is a great one with lots of career options and can be a great way to get the residency and career that the student wants. It is also a highly competitive path. I'd urge you to pick 4-5 schools that have MD PhD programs where your DS would like to do the PhD and look at their requirements and suggestions for when to take the MCAT. Looking at the Univ. of Washington page ( see it here:
https://mstp.washington.edu/mstp-admissions/admissions-overview/ ) they come right out and say "students who most enjoy and are most successful in our MSTP and thereafter are those with prior breadth and depth of research experience, clinical exposure, and community service." They tell you that all requirements should be completed before applying, Although it is a separate application, they link to the MD program to discuss the prerequisites and MCAT. Although you can find the admission MCAT scores on the MCAT website, UW provides a helpful grid of grades and MCAT scores here:
https://www.uwmedicine.org/school-of-medicine/md-program/admissions/mcat To your actual question, UW reminds you "Many applicants take the exam during Spring of the year they apply to allow themselves time to retake the exam if they are unsatisfied with their initial scores. It is advisable for applicants to take the MCAT after they have completed all their science prerequisites. Please note that any previous MCAT scores are visible in your application and will be considered as part of the holistic review." Many of the MCAT test prep centers offer low cost practice tests and I'd urge taking one to see where the student is in preparation. When applying for MD PhD, it truly is "legs of a stool" GPA, research lab experience (not in classes), direct patient contact, and medically oriented volunteer work.. Lastly, the MCAT and GPAs for applicants and matriculants can be found on the MCAT website here:
https://www.aamc.org/media/6151/download Almost forgot to say that many schools limit what PhD you can pursue, so that is also something to check.