Whichever breed you decide to have try to meet a few in their home to make sure that your son isn't allergic (take medication to be safe) it would be awful for all concerned if you bought a puppy only to find that it made your son ill and you had to rehome (hopefully back to the breeder ;) )
If you do a search on this forum there are a few threads about low allergy dogs. Then a visit to the Kennel Club website
http://www.the-kennel-club.org.uk/ will help you narrow it down.
Then visit shows and breeders to make absolutelly sure :)
Hope this helps
Dill