Jules Csillag, Speech Language Pathologist and Learning Specialist
Hi there, I do not work at the Hayfield Montessori School, so I do not pretend to answer this question on their behalf. However, I am a learning specialist who works at a Montessori School, and as such, I feel that this question is in my wheelhouse.
Class size may have an impact on a student with ADHD as there may be less noise or visual distractions (ideal Montessori classrooms have lots of space and opportunities for quiet work), which could help your grandson. In addition, a smaller class size (which Montessori schools also tend to have) may mean that your grandson could get more individualized attention, which he may need in order to begin/continue/finish his work, or implement an academic or behavioral plan. See if the school has learning support staff (e.g. a special educator, a school psychologist, a learning specialist), as these professionals can help train teachers in potential classroom accommodations for your grandson. The school's website currently lists no such professional, but teachers can also have been trained in special education, so reaching out can give you a better idea of what they can support.
Students with ADHD often need classroom accommodations and explicit instruction about organization and focus, but when medication works, that medication allows children to be able to learn those strategies and use those accommodations, so it's not necessarily an either/or situation (though I'm not sure if that's what you were implying).