Culturally, there are arguments for studying all the languages you mentioned; what matters will be where your son wants to live or work, and in what field. If he shows an interest in diplomacy, or senior levels of business requiring access to government officials, French has a stronger legacy than the other languages you mention. German is a widely spoken second language used for commerce. Likewise, if your son shows any entrepreneurial or social enterprise interest, Spanish would be a strong bet given that the global population base and growth rate of mother tongue Spanish speakers is considerably larger than the same for the European languages.
As an American, I'd pick Spanish or German, if your son shows any interest in working in Europe. As English is an indo-Germanic language, his rate of learning German will likely be faster than for a Romance language. However, Spanish is a relatively easy language to learn because it is so regular, which is important given his relatively late start.
As a practical consideration, I'd also consider what immersion opportunities your son would have available outside the class, as that's the way language is learned best. Are there second language clubs or cultural associations for the languages where you live? I'd assume a few hours a week-- maybe 5-- of immersion to become proficient.
