Agree with Dancing Hippo, what's he like after you have gone?
Montessori is really a wonderful method, my DS has benefited so much from it since his horrible isolated start in life when we were overseas.
5 days is really too soon, TALK to the head of the school. The staff have seen worse and better than your DS in settling, let them lead the way, at least for the first term. Ask them for ways that you can help him settle, ask them what if anything you can be doing.
It's OK Char, he'll settle, I promise. TBH, Montessori is such a calm environment, he's more likely to settle there than anywhere else. Stick it out, it'll be worth it.
Once he has made a friend or two, he'll be racing off to nursery!