I'm sorry you are going through this. There are child cares out there that specialize in children with behavior issues. Your on the right track to have your son evaluated by the school district,
The Montessori enviornment is a hard enviornment for kids. When they are in work time they are expected to be pretty quiet. Kind of hard for 3's and 4's to do but they do accomplish that. A child who is not in sync with that is considered disruptive.
Of course your son behaves better at home. He has just you and Dad, no other children there, or at least not a classroom full of children in his age group to compete with. He has your attention, his stuff, his enviornment. It's different at home. Plus parents tend to over look things because they become accustome to that behavior..
When you call providers ask them first if they are capable of handling a child that might be a little challenging. Get a feel for their philosphy and thoughts first. Tell them some of his traits and see what they think. Let it be known how many classmates he had too. Sometimes a small group might be less overwhelming to him.
Save the part about being kicked out of school last. In fact see if you can interview and visit their program and let them meet your son in person and draw their opinions from the meeting. Telling them right off the bat about being exspelled just paints a tainted picture before they have a chanc to know him. What area do you live ?