I'm glad you are concerned about his well-being during this transition. The differences between Montessori programs and Kindergarten is that Montessori is an alternative method, more play based, focusing on self-directed activities with teachers being facitalitors. Kindergarten is more like school with planned learning activities and less focus on individual play. It's a big move and one that may cause stress if the child is sensitive and or having a hard time adjusting.
The following is an Education.com reference article based on transitions