Check out the credentials of the Montessori school. If the instructor your child will have has been certified personally, ABSOLUTELY YES put him in.
Our oldest attended www.skinnermontessori.com for 3 years (when we lived in WA state), it was the best investment we ever made. We paid $5650/year and then also paid for before and after school care as well (about $900/month total). Garret was dx with a developmental delay in speech at the age of 3, began attending Skinner at age 4 1/4 and yet he still was able to read at the age of 5 1/2!
Today he is a 7th grader in public school, and in every advanced class they can give him (Science, Math, History, English, Reading, even Orchestra). If the montessori school is using a learn by phonics method and teaching a foreign language (Garret learned French), that is usually done by phonics as well. Garret is academically ahead of almost every child in the 3 different states we have lived in (attended public school every time), but he has been more confident with every move we have made because of the excellent base of education he received at montessori.
The only problem he had going from montessori to public school was a noise and chaos issue. Montessori is very quiet and calm, public school is NOT. I just had meetings with the public school teachers he had and let them know the environment he came from (montessori) and they would be sure to move Garret to a front corner where it was a bit more quiet to be more conducive to his learning style.
I wholeheartedly endorse certified Montessori schools. We figure that you only get one chance to teach your child and we have absolutely no regrets on the money spent.
Also, read about Maria Montessori and how she sharted, it is amazing. I even looked at getting certification after my BA ~ that is how impressed I was with the school. If you have questions, call Nikki at Skinner from the website above, tell her Garret's mom sent you and she'll be able to answer your questions regarding certification, there are many types.
Our middle child, a girl, also attended a Montessori school in FL when we lived there and only the owner was certified, but our daughter loved the school and although the teacher she had hadn't been directly certified, it was good, but not GREAT like our first experience with each teacher being personally certified.
***I am editing this quickly: I just read all the other responses, but AFTER I posted, amazing how similar the advice is. We have 3 kiddos, ages 13, 5 and 3.