D.M.
Lots of good points. I have a 2 1/2 year old little girl and have been starting to think and research some similar choices when she goes to kindergarten. She's currently at a Primrose (which I love).
My two cents - keep him where he is Goddard is a great place, and he seems happy and is learning and thriving. Don't make the change until Kindergarten or 1st grade. Montessori can be good - depending on the school and the child - all about fit. My concern (before I read far enough) is that at 2.5 it's too young to be expected to interact with all ages. I think the more focused and fun approach of Goddard is the way to go.