I put my daughter in montessori school at 20 months (she is 23 months now) and I am glad I did. She goes 3 days a week. It took some adjusting on both parts. She does not get enough sleep at school and it has taken her some time to get used to that; some days she is still really fussy in the evenings after school from all the stimulation and short nap. I had to alter her schedule a little so that it would be more like the schedule at school.
She has never started biting, but she has hit me once or twice. I don't know if she gets this from school or if it's just a 2 year old thing. Her vocabulary is growing, she eats with a fork and spoon now, uses a napkin, and refuses to use a sippy cup at meals, she wants the lid off! She can put on her own pants now and asks to use the potty sometimes. She understands the concept of sharing and taking turns and she puts up toys when she is through with them. She cleans up her own spills and likes to help me do whatever she sees me doing. I use my days off to run errands, shop, craft, and clean house so that on the days she's home, I can just spend fun time with her.
If you start him in school, I would do it well before the baby comes so he can have your full attention while he is adjusting.
Good luck with the new baby.