Best dog breed for ASD/PDA child?

PS: Jean Donaldson's 'The Culture Clash' is a great book for understanding your dog and avoiding problems.  I do think they are sometimes capable of more complex thought than she gives them credit for, but using this book as a guide will help you deal with your dog when they are being a 'normal' dog!

Dogs are all individuals, so there's no such thing as a breed that can be guarenteed to be right.  Early enviroment and training also is a big influence. Guide dogs have been breeding their own dogs for years but they still have a high drop-out rate (though some of their rejects do go to other support roles, including autism support).

Had a quick look, and it looks like the charities who provide support dogs have closed waiting lists at the moment, though you might want to check this yourself (search 'autism support dog uk' or similar)

I think the first thing you should do, which does relate to breed, is consider what you, as a family, can provide for a dog.  Can you provide half an hour or several hours of exercise for the dog?  Can you take it to training classes? Do you have space and food budget for a big dog? (Since your boy likes to hug, you probably don't want a small, fragile breed like a chihuahua...).  Can you cope with the grooming some coat types need? (May mean daily grooming and/or frequent trips to a grooming parlour). Have you got a breed/look you've always wanted?  What breeds does grandma have? If you don't want one of them, why not?

A breed selector, such as this one http://www.thekennelclub.org.uk/services/public/findabreed/ can help you get a better idea.  Once you've done that though, also make sure you research a breed and find any tendency to health problems: many breeds are inbred and have frequent health problems as a result, which can end up being expensive.

Once you've got an idea of the type/breed of dog that would suit your family (eg medium sized, medium exercise, short coat...), then it's time to look for the right individual.  I'd recomend that you get an adult dog from a rescue. With a puppy you can't guarentee what the adult dogs temperament will be, and 'accidents' in socialization (such as getting a few scares from your son...) could easily cause things to not turn out well.

If you have decided you really want a particular breed then many breeds have specialist rescues.  Otherwise a large rescue should be able to help you choose a dog with the right temperament.  You want a dog that has proven to be resilient and have good 'bounce back' when faced with the unexpected and loud noises.  A good rescue will know/be able to test whether a dog has this.  Next is meeting a candidate dog or dogs: it's no good choosing the perfect dog if they don't 'click' with your son. Finally be prepared to take a dog back if it doesn't work out: you can't 100% predict how the dog will react when faced with repeated meltdowns.

Once you've got the dog home it will help if you make your son as positive as possible for the dog: that means whenever possible the dog should get things it likes around him, such as games, walks, treats and meals.  But take note if the dog shows signs of avoiding or ignoring your son: these may be early signs that the dog is not coping.