A huge factor mentioned in later posts- distance to school. The IB school seems to be a lot closer. Time spent travelling should be minimized.
The IB school seems to have better academic offerings and activities.
He can get any certifications in other ways than needing the immersion school. He intends to go beyond a basic college degree (eg law at this time).
I do not see where after having enough Spanish to help you out he needs the extra immersion at the expense of academic rigor. Spanish only helps with Spanish speakers, as mentioned by a poster long ago it will not give an edge for military purposes. There are so many other languages that English proficiency is most important- so people from all over can communicate with each other in this country.
Two big questions- does he have a strong preference and is he competitive for the rigors of the IB program. We don’t need the answers, but you and he need to discuss the pros and cons. Your son wants to be best prepared for a top tier college, rigorous coursework will prepare him best. Knowing a second language will not give him an edge.