My question wasn't answered but here are my thoughts anyway. I gather from your title and the hints in your post that you're concerned about potential problems between the dog and your son. You are smart to be thinking about this.
It's less about the breed of dog than it is about the personality of the dog. With that said, some breeds might be nippier than others, such as herding dogs or toy breeds.
Whatever dog you get, both your son and the dog *have* to be taught mutual respect for eachother. *That involves constant supervision on your part.* Your son should never be rough with the dog, and the dog should never be rough with your son. I can't stress this enough, especially if your son has never had a dog before and doesn't understand the etiquette of interacting with a dog.
Good Owners, Great Dogs is an excellent book for your own library. The same authors also have books specifically about dogs and kids together.
Only you know your exact situation, but you should choose a dog that's fairly hearty if lots of physical touch will take place.
Tug of war type games that pit dog against human should be discouraged - stick with fetch and hide and seek type games.
And lastly, make sure the dog has a space of his own that he can retreat to and be left alone when he wants. He should never feel "cornered" by a child that won't leave him alone.
It behooves every dog owner with children around to learn the "anatomy of a bite" so that common sense can be used and warning signs can be recognized.
If it were me in your situation, I'd want to pick a dog that was perhaps a young adult to see what the basic size and personality/temperament were like. This doesn't mean that problem behavior can't be learned, but it's a good start. You might want to look for a dog that's been raised with (and proven to enjoy) kids already.
Good luck.
http://www.peteducation.com/article.cfm?c=0+1278+1495&aid=628
. My childhood beagle was a downright killer of bunnies. Others have just been chasers. 
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