I recommend adopting an older dog as well. We adopted a 3 year old lab that is very mellow for a lab. She is sooo sweat and everybody comments on perfect she is for our girls. She is ready to snuggle or play when they are.... Yet can read their cues of when to give them space. It is funny I believe she knows before I do.

An adult retriever. They are very loyal. If you go w a puppy just know you will need to put in the time to train it.

Yes, I agree with some of the above comments.
I am a strong believer in Shelter dogs of a mixed breed. I have a 15 year old dog who is a mix between spaniel and border collie. Mixed breed medium sized dogs of about one year of age, will hopefully be house broken, and have less health problems than a pedigree dog.
At have humane society shelter you can meet with the animal to see if they bond.

They say Labs are the best and if you look at the service dogs for special needs kids most are labs. Plus later you can have it trained to be a service dog.

We have two Golden Retrievers and they are the best with our son. They are very tolerant and laid back, which allows my son to interact with them in his own way.

I would suggest looking for a similar breed. Labs, Goldens, etc. The best thing to look for is how the dog responds to your son. If you watch the dog you will know if it is a good fit or not. If you are able to have the opportunity to interact with the dog a couple of times before committing that would be best, but I also know may not be possible.

Good Luck! Having dogs has been great for our son. I'm actually considering having them certified/trained as therapy dogs.