The only way to get college academic credit for a dive class is to take that dive class from an academic program. That might actually be at a dive shop through an agreed upon arrangement between the academic program and the dive shop, or it might be an actual academic class at the school. But, as you are finding, you aren't going to find a school that will give credit for a previously done, non-affiliated dive class. Just like they aren't going to give you credit for a non-affiliated swimming, welding, cooking or 3d printing class.
The PADI statement is just saying their academics are rated as rigorous enough to count for academic credit by the ACE. That is to help the PADI instructor at the dive shop develop those arrangements with a school, not for individual students to get credit at random universities. The fact that it is often misconstrued may or may not be intentional.
My university has a full recreational program from OW through DM, technical diving and scientific diving. We have a diving minor. If your son came to us and wanted credit for classes he already took (we get this question all the time, btw), we would say we can't give you academic credit, but we will recognize the certification and previous experience. You don't need to take OUR OW class, your PADI one is just fine, it just won't count toward your degree. You can go ahead and sign up for the advanced (or whatever) class that WILL count toward your degree.