Prepare them by talking about going to Montessori school in September. Discuss the other children he will meet, friends, neighbours, cousins, that there will be lots of new things to do.
Tell him that you will be doing some ordinary activities, i.e. working, shopping, tennis or golf. Try to make sure it is something you usually do without him, so that he won’t feel he is missing an activity you usually share.
Please do not over exaggerate the excitement of Montessori school either. School is not Disneyland, so be realistic when discussing school with your child.
You might like to talk about how he will begin by doing puzzles, colouring, painting, playing with play-dough, singing, reading stories etc. These are simple activities he will be doing for the first few weeks.
Simple tasks and familiar toys are re-assuring for children in the early days. Once the children have settled Montessori materials will be introduced little by little.