Montessori pretty much as you describe - setup, advantages and drawbacks. The other drawback is there is no awful teaching, in line with the tenets of Montessori philosophy, so whilst your child may acquire the foundation skills being taught in maternelle there's no guarantee.
EAB(& EABJM from MS) are primarily in French with a French teacher (who is well used to having non-francophones in the class) and approx 1,5 hours in English with an English teacher. At EABJM this is streamed, not sure about EAB but I suspect it is differentiated in some way.
Le Petit Cours de Rocher I'm not sure what they do. Their website looks like the same set up linguistically as the Montessori, but maybe with short periods of exclusive English.
As for which would be better only you can decide but something to consider: if you're staying past the age of 6 and you choose to stay at the Montessori until then you face the related issue of whether you want your DS going into a bilingual environment where one language or the other is spoken (and a higher standard expected than PS) but reading/writing isn't expected, a totally francophone environment where reading/writing isn't expected or an anglophone environment where it is. If you may stay long term I would focus on language acquisition to give you more flexibility on schooling at CP level.
Personally for a child up to the age of 7 I would go for the immersion approach with the aim of encouraging bilingualism every time and IME it's easier for the child, after the first week or so, if the languages are clearly separated out. Remember at the age of 3 their native language is still being acquired too and hearing both languages may be more confusing than reassuring.
I think your son will always be able to communicate to the teacher in English. The difference is with a dual setup the English teacher will respond in English, so he may develop a preference for that teacher and stop attempting to communicate with the other. Where the language in use is French the teacher will understand what's said but at first paraphrase into French and respond in French, then ask him if he can say it in French, prompt attempts, model the correct form and answer, so there shouldn't be any frustration (which there could well be at an exclusively francophone school where the teacher may just not understand) and your son will quickly acquire an understanding of the language, followed by an active command.
That seems very ramble to me but does it make sense?