Since you haven't received many answers yet, I'll tell you about our experience (although not with an immersion school). My son just turned 5...and is trilingual. Multilingualism has been a gift for us, as it is keeping his brain busy (or at least busier than it would otherwise be). He is now reading in 3 languages, and certainly beyond K level in at least two of the languages (we just started reading in the 3rd language more seriously a month or so ago - and he is quickly catching up). I am not sure how long immersion will keep your LO busy (languages come so quickly to kids this age, and probably especially to our kids), but it would likely be great for a year or two. I'd probably start with the immersion school, and move on to the gifted school once that starts to fail (which I suspect it will, unless the school is open to accomodations), and then reinforce his language skills at that pint via meetups/ Saturday classes for native speakers or summer camps etc.
