That's what we are doing.
My son is too old for the charities that have support dogs for autism (young adult) but desperately wanted and needed a companion.. in fact the only thing he has ever asked for is a dog.
So we got one (obviously it wasn't as quick or unconsidered as that)
Went to Discover Dogs at Crufts to get a feel for different breeds, then went away and researched a short list.. then met a couple of breeders and went on a list.
Puppy is now 7.5 months old and has transformed my sons life (and mine!) Just normal training, but we chose a breed that is devoted to family and she is just THERE for him. He walks into the room and her tail starts wagging and my son feels loved. (He'll never marry or leave home as he also has learning disabiltiies)
Once winter is over we plan to do more advanced training with her, but actually the thing he needed..companionship, cuddles, someone who didn't care that he walks oddly, talks strangely.. she provides that now.
A caveat tho.. a 6 year old may THINK they want a dog, but the reality might be different. If you are thinking of a puppy (and frankly colour should be the last thing when deciding).. puppies bite, puppies poo, puppies cry, puppies ruin kids stuff... they aren't remotely useful at first and can be basically pooing peeing sharks! They are SUCH hard work at first.. can you commit to walking a dog several times a day (who will look after your son when you are out at 6am, or 10pm and it's pouring with rain etc),..
We adore our puppy and she has added hugely to our lives, but the committment is HUGE... can't go out unless we have dog care etc.
My son was old enough to be able to take these things on board but a 6 year old would not.