Just a little note on placement tests: DD was in a situation similar to OP’s son and took a placement test for Spanish II when she entered (Catholic) HS from secular middle school. She didn’t pass, and when I asked why she thought tha happenedt, she said there were cultural questions she had no idea how to answer. It turns out the placement test was the Spanish I final, and there were several questions about sidebar topics the text. We bought a cheap copy of the text, she familiarized herself with the sidebar content, and did well enough the second time to place into second year Spanish.
That said, I think retaking Spanish I is a legitimate choice to improve foundation and allow son more time and effort on other 9th grade subjects.