@Ynotgo – Either the Bio subject test is relatively easy or it aligns well with the 9th grade honors bio curriculum at our HS. My son, and several of his classmates, took the Bio subject test after 9th grade bio and scored well. Very few attempt the Chem subject test after completion of 10th grade honors Chem but instead wait to take AP Chem the following year, and take subject test at completion of the AP course. This may just be specific to our HS curriculum as I have never even looked at the content of the exam, or our courses.
Having said all that, IIRC, your son is interested in coding and CMU specifies with science subject test they require for each college, and Chem & Physics are listed more often than Bio, so your son is probably all set with his 9th grade Physics scores.