Aw I'm sorry that's tough to deal with! Your poor son too.
Have you tried to have your son walk into the room without making eye contact, just walking in and maybe dropping a few treats by the dog, and then he walks and sits somewhere else. See if you could get your son to do something like that a few times just so Marleau can see that son entering a room = good things happening.
^ I was typing this when I saw the post above where you got great advice.
Here is a good youtube channel and here is a specific video on shy dogs:
https://www.youtube.com/watch?v=NOjIFYvItAY