I would say the critical question is how many students are accepted into the OT Masters or Doctoral Program from the OT focused undergrad program at a particular school.
It sounds as there is the opportunity to transition to the graduate program at various time points in their undergraduate program. But what percentage of those undergrad students actually are selected? And what are the selection criteria?
I am not familiar with these programs. But I do know (as a PT and Rehab Director) that PT and OT grad programs tend to be very small (less than 50 students) as the school needs to have clinical opportunities, faculty and mentors in the surrounding clinics to support student practicums. Thus, they are also quite selective. I know back when I applied the acceptance rate was around 10 percent. I will say there are many more programs now compared to when I applied, so I would assume the acceptance rates are higher. But in these combined degree programs, it would be helpful to know the acceptance rate for the grad OT program.
I do feel that Allied Health grad programs look for students who have good academics, but also show a passion for the field and an understanding of what it entails. Your son certainly seems to demonstrate passion and understanding.