Does your Montessori school end at K or do they have elementary as well? Perhaps the teachers would be willing to let him just keep working ahead with the materials? My 6yo is still in a Montessori class for K and the teachers just find more advanced materials and projects to accommodate her. In fact, we're moving her to a Montessori elementary as that seems to be the place she's likely to get the most individualized curriculum (outside of home, of course).