I will second the rescue idea, most of them will give a two week home trial, we have a rescue staffy and she is the greatest with my 2 dc, she is happy to be left alone to sleep or is happy to be dressed up and used as part of a game she is my 3rd child
If your local rescue won't rehome with you, I would worry about your set-up. I'd imagine it would be very difficult to have a dog - especially a young dog still going through toilet training - without easy access to a garden or private area of ground.
I think a golden retriever or a Lab would be a good match. Breed rescues tend yo have fewer set rules and will assess each family as individuals. But do consider a retired greyhound too. They tend to be very steady, loyal and easygoing.
A friend of mine has just rehomed a guide dog who failed it's training! Obviously still an incredible dog, but just a bit too shy to be a confident guide dog. Lots of labs, golden retrievers. Might be worth looking at.
We had a pup and struggled with toilet training because we couldn't access our back garden easily. Walking several times a day sounds easy, but it prolongs the training process in my experience and your dog may become very demanding, as ours is, when he/she wants to go out. Rabbitsdog is 'trained' to go for walks several times a day now, even though we have a great garden in which she can go and she's an itty bitty dog with a lazy streak three miles wide!
Having said all that it was worth the misery (in retrospect and now we've got past the 'it was your idea to get a bloody dog' divorce threats) and I wouldn't be without her.
Don't whatever you do, please, buy a dog from gumtree...no ethical breeders advertise on gumtree, it's a cesspit of puppy farmers, back yard breeders trying to make money from their pets and people trying to 'get their money back' from pets they've bought and couldn't be bothered training and now can't cope with.
Basically people you don't want to be financially supporting for bad choices.
You're spoilt for choice with rescues, there's no shortage of them and plenty of them are willing to properly assess whether you're suitable for a dog rather than having hard and fast blanket rules.
look at https://www.facebook.com/BlackRetrieverXRescue?fref=ts if you have Facebook. I also second those who advise you to go for a slightly older dog. The cute puppy phase doesn't last long but is such hard work (look at some other threads about puppies on here and you'll find people tearing their hair out). Black Retriever Rescue has some gorgeous dogs, they are in foster so have been assessed with children/cats/other dogs and therefore would be perhaps a more reliable choice for you.
I'm sure you've thought about it, but a retriever/lab size would be too big for your 9 y/o to take for walks, perhaps a smaller size type would be easier for him. Smaller dogs take up less room generally, easier to get in a smaller car etc.
Also think about costs of vaccinations, kennel/dog sitting if you want to go on hols without dog, etc. Long days out are difficult too unless friend/relative will come and take dog for walk... and your DS will love to take dog for a walk sometimes but main burden of looking after him/her will be up to you.