First, you may need to request to have a middle school AP score "kept on the record", and I think AP scores are only valid for five years.
That being said, I don't think very many colleges give credit for Computer Science A, and if your son is big into STEM, he will certainly have more impressive CS accomplishments that he can list on college applications.
I wouldn't worry about gaining HS credit. Getting enough credits to graduate is typically not a big deal. I would ask what you mean by "all the core AP science exams" and along with "all the mandatory courses and electives". My middle kid likes to learn about anything and everything, so she took a mix of APs on the STEM side and humanities side. In the end, five STEM and five humanities APs. And the kid still found time for Netflix binging (along with two varsity sports, and travel in one sport).
I say take the course and the exam, as it gives him a good introduction to APs without any pressure. I would focus more on crafting a path through HS that meets his interests.
