One thing about doing "only those things that you like" is that, in Montessori, the curriculum is so integrated that what you learn in one subject extends to other areas. When he starts doing research for different projects, you will never have just one area he will look at. The teacher can easily ask questions for him to look at to see the other areas. Let me give you an example.
Let's pretend your son wants to do a research project on Taiwan. In traditional schools, that would be given in one class - geography. The child will write about it from a geographical point of view and only discuss it from that area. He gets the paper back, sees he got an "A," and thinks that is all there is to know about Taiwan.
In Montessori, he begins by getting whatever research materials are available to him. These might be books, internet sites, magazine articles, even the old encyclopedia that (thanks to the internet) has left many door to door salesmen homeless. (Which is probably why we see more Mormons these days)
: (Side note: I do like Mormons. They tend to be very nice people. So don't take that as an attack of Mormons...just a silly joke)
He gets these research materials (not materials just given to him by the teachers, but ones he seeks out). He begins to read about Taiwan. As he writes his report, the teacher sees he has completely ignored math. Rather than force him to do math, the teacher encourages more study on this subject that he is already interested in. She notices he mentioned two common languages in Taiwan: Mandarin and Taiwanese.
"What percent of people in Taiwan speak Mandarin and what percent speak Taiwanese?" The child does not know how to figure out a percent, but has all the data. So now the teacher can give a lesson on fractions and percentages. The teacher can also give a lesson on how to make graphs and charts to portray the idea.
This one research project can lead to so many questions related to the rest of the curriculum, depending on his age level:
--What types of rocks do they have in the mountains? (Geology/Science)
--What are the two main political groups in Taiwan? (Government)
--What are the different counties in Taiwan? (Geography)
--How do they celebrate their religious festivals? (Culture/Religion)
--What problems does Taiwan currently face? (Various subjects)
--What different types of food to they eat? (Cultural)
--You said this even happend in 1958. How many years ago was that? (Math)
--Are any countries near Taiwan posing a military threat? (*cough* China *cough*) (That will lead them to look at both government AND Geography)
--Can you draw a map of this part of Asia and label the countries near it?
--Those countries that are near it, how much bigger or smaller are they?
I could make an extensive list. The key thing is the way the curriculum is interconnected and the fact that these concepts can be taught much easier with the materials in the classroom. The fraction materials are as genius as the other math materials. The geography maps are there, so they will have a good sense of geography. The child has an interest in learning more about this subject. It was chosen by him. It may be different for another child - frogs, a famous person, a great book....something along those lines. The teacher can integrate so much into the research project that it is impossible to say that the child will walk away not learning anything that he needs to know.
There was one time in elementary where, for a long time, all I wrote about was Techumseh. It was getting to the point where I was just not engaging in it any more. My teacher, observing this, said, "I think you did a great job in this. Would you like to try researching something else as well?" I knew he was right, so I agreed.
Let me also add that the materials for Elementary are just as interesting as they are for 3-6. I loved making square roots with the stamp game or working with the trinomial cube to figure out advanced math concepts. The geography takes on a different role. In the Primary years, it centered a lot on knowing the names of countries, land forms, and understanding different cultures. In the Elementary years, I vividly remember 1st grade and learning about different crops each area grows, different current events that are happening in the world, and developing more of a global perspective. I remember this in the 6-9 age group, where many schools start talking about current events and cultures on the same level much later in schooling.
The biggest difference you will really see between a Montessori School and a Traditional School is the philosophy. In a "regular" school, the philosophy is built around an idea that a child is a blank slate and the teacher must give him knowledge or he will not know anything. Montessori believes that children can develop intellect in the right environment and the teacher can help guide the child to develop that intellect.