When I was a child (about 7 years old), we had dogs. 2 dogs at the time. One day we came home from school and 1 of the dogs was gone! Just like that. My Mom simply said that 2 dogs were too many, so she gave it away.
We were angry. She gave no consideration to us... and she didn't talk about it with us. She simply made up her own mind, and got rid of our dog without our knowing. And, she never said sorry for doing so. We, had always remembered that... and thought it was so mean of her. Even my Dad hadn't known she was going to do that. We didn't even get to say 'goodbye' to our dog. Anyway, she didn't do that again, with our other pets since then.
But, the moral is... you have to be honest about it. Your son is still young. But I would explain that, as the others said, that perhaps Mommy made a mistake... the doggy is simply too big and too big for the house. Newfoundland's are so cute... but, to be happy, it needs a BIG house and a BIG yard. And that in the future, you and he will choose a pet that is suited JUST for him. Special and a good fit.
Perhaps next time, as he gets older, or when you feel he can handle it and the proper handling of a pet... get him a pet that is a good match in personality. Even perhaps, not a puppy, but a grown dog, that is well suited for children. Not all dogs like children, or the rough-housing.
I wish you the best... I know it's not an easy position to be in. Hopefully, your son was not heavily 'attached' to the dog? Or was he? Sometimes, children get over something like that quickly, so you never know. But allow him to talk about it whenever he feels like it. Openly.
And i feel, it's also important to apologize, say "I'm sorry..." to him. Acknowledge his feelings too, and say you are 'sad' too....but it's important for the dog to be happy in a good home etc. And that even grown up's make mistakes too, sometimes.
Once, we had a pet rabbit for my daughter. But it was destroying our house so we had to get rid of it, and adopted it to someone else. Our girl was so sad... but we explained that the rabbit was not happy and it was destroying our house... and when it was okay, we would get another one for her. She still talks about her previous rabbit...even though she has another rabbit now (which she loves). But we let her talk about it, and she still loves her previous rabbit and has her memories of it still. It is bittersweet.
All the best,
Susan