I agree with Kriston. You have already made the decision to homeschool once. This comes with the same issues regarding friendships that the private school may have - more as there are no other children of any age. Also - maybe there are other parents who are waiting to join the small private school like you - if more elementary students join.
But as you say - maybe DS will fall in love with the Montessori.
Personally I am jealous as it sounds like you have three options - homeschool, montessori or private school. That makes you pretty rich in choices in my book! In the end you will make the right choice. And sometimes the right choice this year is not the right choice next year.
Let us know how DS likes the Montessori school!
Breakaway