Chay makes some great points below!
We also struggled with this decision, but were choosing between FI and a congregated gifted (English) program. I think if the choice were between FI and regular English, FI would win hands down.
If you are able to get an IEP, that will also help depending on the teachers. We lucked out this year (grade 2) and had both English and French teachers willing to work with us and differentiate.
Both my kids are enjoying FI and have found good friends among their peers.
On the other hand, if you see value in the English program your DS can probably learn French later - my DH learned French in high school and university (and did a couple of summer immersion programs) and is now fluent. So, not all is lost in choosing English, especially if you speak French at home!
