Once you are involved into the CASA program, you have to stay for 3 years to be graduated, and then go to elementary in the same Montessori school. The graduates from CASA program are equivalent to grade 2 in the public school system academically. So you would have a long term plan, better to put him into at 3 yrs old so when he finish the CASA at 6 yrs, you could choose between either public or private schools, in both cases, he will have some academic advantages.
The CASA starts at two and a half (Entrance age 2.5).
If i were you, i would wait until he is full 30 month, and start CASA, which is a very very good program for this age.