Hi Redmen
Some of the ideas above are superb, As Tilly Said Pets As Therapy Dogs are great another good site to look at would be
http://www.safetyarounddogs.org.uk/ the scheme promotes safe interactions between children and dogs, you would like any leaflets i have plenty to give.
I run Champion Dogs with my partner, as part of classes we run we have a puppy socialisation class, its only a short class so young dogs can interact. We would welcome you and your son to come and watch the puppies play from the safety of our unit behind a gate to start with, as your son becomes more relaxed we could borrow a puppy for him to meet and eventually have you and your son with the puppies as they play.