Maybe I’m missing something here, but why is it a problem to have his college classes listed on his HS transcript? IIn principal this is no different from taking a HS AP class. That appears on the HS transcript but many colleges will give give college credit for it, at least with an acceptable score on the AP test. Many high schools have “college in the high school” programs where students take one or more college classes as part of their HS curriculum; many colleges will transfer credit for these, provided they think it’s a legit course and the equivalent of something they offer. As far as I know, those courses end up on the HS transcript, and that doesn’t prevent the credit from transferring. Here in Minnesota we have a program called PSEO (Post Secondary Enrollment options) where the state will actually pay for qualifying HS juniors and seniors to take regular college classes on the college campus, for credit; as far as I know those PSEO courses routinely get listed on the student’s HS transcript, simply to indicate that the student was a full-time student between the regular HS classes and the PSEO classes. (I’m not sure about that, though, because we homeschooled which meant our daughters’ junior and senior year HS classes were almost entirely PSEO classes at our public flagship, but we certainly listed them on their homeschooling HS transcripts to indicate how they spent their junior and senior years), I think you’re looking for a solution to a non-problem.
If I were an admissions officer, I’d look more askance at a senior year transcript that showed only 3 classes per semester. Then it looks like the student has a serious case of senioritis and is just coasting through senior year.