Just so you know you can spend 1 full day to see the US sector and not see everything.
I had a personal guided tour using his car and didnt see everything. He knows the lay of the land and where everything was so driving youself can be done, but i can bet you, you wont see as much as if someone is going to show you. There are +/- to DIY vs tour so think about how you want to spend your time.
Since youre in the planning stages you have time to do some homework and figure out your time. Look at the tours suggested and go from there. They give you an idea on where they are going. IF you dont see what you like then you may have to go it alone or private.
There are also several good museums up there and it would a shame to miss them.
there is also Mont St Michel that is really cool, but thats a day trip all by itself.
you can take the train up there from Paris. I also saw tour busses leaving Paris for the Normandy Beaches too. But consider the time back/forth.
Just so you know, the train i took from Paris to Bayeux was more or less an express with one stop in Caen. that trip took me approx 2h.
Knowing what i know know, i woudlnt buy my train tickets early. yes you can save $$$, but it locks you into your day/time. If you want to know the savings/cost, just plug n chug some dates into the web site. Note that iirc, you can only book 3 months out, but pick a similar day.
also, if you son is old enough have him do some research (homework) so HE can figure out what HE wasnt to see. Why should you have all the fun?
btw, this is just my opinion. But planning it yourself upto a point will make it less stressful once you get there since you will know all you need to know. If something goes wrong, you will know what you did to get where what you want.