L.C.
My five-year-old preschooler has a classmate, "Tom", who behaves the same way that you describe. Tom has development issues and has been in PT and OT since he was two years old. He can be pretty aggressive and disruptive in school, but he is a good kid, and my son likes him. He has an IEP, so after preschool, he goes to a public school for PT and OT.
I know all Montessori schools are different, but have you ever thought of enrolling your son in a preschool with more structure? My daughter briefly attended a Montessori school, and she felt like she was drowning there all day. The kids weren't encouraged to socialize or foster friendships, and the lack of roleplaying toys (like dress-up clothes, play kitchens, or stuffed animals) made it hard for my daughter to find ways to interact with the other kids. I felt like some of the children with behavior problems were allowed to get away with their bad behavior, because it just wasn't addressed. As soon as I moved my daughter to a more traditional preschool, she blossomed and bloomed. Apparently, she thrives in more structured environments.
Before you look into play skills or social skills therapy for your son, ask the staff at his school how they can help. If they don't seem to be interested in working with you to correct his behavior, you might want to shop around for another preschool. You want to send your son to a school that can be more proactive about handling kids with sensory and social issues. Good luck!