He will be in the Montessori 1st-3rd classroom, right? Obviously they are not full if they accept him now. Chances are very good that they won't be full next year either.
Of course, they would like you to make the decision now, pay part of the tuition, etc, but if you postpone your decision till August they will probably be very happy to take him back regardless. As long as they are not full I don't see a reason to make the decision now. The same is likely to be said about the other private school.
Don't let push you into making a decision until you are ready.
