I would focus on two or three languages total and aim for mastery. If he is wired to be a polyglot, he will be able to pick up other languages as he gets older. As he matures, so will the vocabulary and linguistic constructs he accesses, much like progressive overload in athletics.
If languages are his angle, you might consider enrolling him in a school whose primary language of instruction is not English.
A word to consider: your son now has phonemic processing capabilities for languages in the Romance and Indo-Germanic groups. If he has interest in a language with a radically different phonemic structure (Mandarin, Arabic, etc), you may want him exposed earlier. We actually lose the auditory ability to discriminate non-native phonemes as we age. (Patricia Kuehl is an excellent resource.)
In your shoes, I would consider weekend language classes in his favourite foreign language from among the list you’ve provided, so long as he delights in the activity, and a half-half immersion school in English and Spanish during the school week, given that it sounds like he’s had the richest exposure to Spanish.
Practically speaking, Spanish, English, and Mandarin are the most extensively used languages of the list. Unless he has a family connection to French, Greek, Italian, or German - and the reasonable expectation of daily practice with a native speaker in those - I would save them until he’s older. His current languages give him sufficient phonemic access that he could credibly learn to speak them with a native accent later.
