With much lower PRI than VCI, you might be looking at a situation where there are 2-E issues, or possibly problems with vision or visual processing. This merits further investigation. A complete evaluation (not a regular pediatrician or optometrist simple eye exam) should help you rule these kinds of issues in or out. Another possible cause for a big PRI/VCI split is a NVLD - non-verbal learning disorder.
The scatter within the verbal subtests could easily be an artifact of the tri-lingualism, and I wouldn't be overly concerned with it unless he also showed some signs of actual real-life difficulty with the lower subtest areas, like social comprehension, for example. The tri-lingualism may also depress the VCI somewhat at this age, still, as he has been busy acquiring vocabulary in 3 languages, not just one. Is he used to hearing the accent the tester had? If he found the tester difficult to understand, that could also have impacted his scores on the vocabulary and comprehension subtests.
The achievement testing is much more in line with his scores on the similarities and information subtests.
If I were you, I would keep homeschooling for the moment, and get more information about what may be depressing his PRI score before I shared the info with the school, perhaps by getting a neuropsych evaluation.
Just my .02 - YMMV.
