Thanks so much for the thoughts! The native lang. school is what we had always envisioned before we realized that he is ahead of other kids his age...Some risks are that they might have him repeat a year sooner or later for behavioral issues if they let him into K early next year (on average, he'd be 1.5 years younger than his classmates). They did mention a case like that. We'd be a bit stuck in that case: Any transition to other private schools would be awfully complicated due to the language barrier (they don't introduce English until a bit later), if this school were to turn out to be a bad fit. I did like the expensive English private - but he'd be in a class with kids that right now are basically scribbling more like his 1 year old brother than himself. Do we risk him regressing?
As for the stay at home option - we'd have to hire a nanny. So far we've been juggling the two on our own, thanks to our flexible schedules. But it will be too much for us soon - it's exhausting especially now that the 15 months old wants tons of stimulation too (he spends hours a day looking at books; both by himself and WITH one of us...). We could enroll him in some afternoon classes etc. to get him used to a more school like atmosphere with other kids.