What a bummer, you had things all lined up and now you are starting from the scratch
May be the Montessori with curriculum up to 3rd grade would be an ok option. Being able to do 3rd grade material may be better than being in the 1st grade learning the 1st grade material. At least you would know what you are getting into.
I hear you on the social issues, we were in similar situation this year. No friends in school, too few kids and most of them younger, forget intellectual peers. The social part didn't work there. DS is doing 4th grade math and the teacher has been coming with new things just for him for most of this school year. Hopefully your son's teacher can do the same for him and if not I would make it clear that you would supply his math material if they run out of things to give him.
I too am not happy with our M experience (mainly because of DS3, DS5 did pretty ok), but it is still our back up option.
Could you do half public K and half Montessori?
Good luck with your decisions.
