A dog can just be a HUGE comfort to an aspie kid. We have a large, loud, excitable Labrador, but my daughter adores him, he sleeps with her every night, and is basically her best friend. When she was hospitalized for anxiety and depression this year,she missed him terribly, and the best days for her were the days the therapy dogs came to the unit (unfortunately only happened twice). I can imagine that some dogs are more suitable for kids on the spectrum than others (maybe King Charles spaniels?), but any dog that your kid grows up with and loves can do major good.
Cats, too, for that matter. My daughter can get through a meltdown by stroking the cat.