Dog to help with anxiety and confidence

My nearly 6 year old high functioning lovely son suffers with anxiety and lack of confidence.  We think a family dog coud really help.  We understand that the opportunity to get an assistance dog is extremely slim and we are not even trying to go down that road.  We have contacted a dog trainer who will assist when we find the right dog and we are prepared to find and pay for a dog ourselves.  The issue is that I like recue dogs as I think that breeding dogs is often irresponsible and no more than an opportunity to make cash.  However, we need the right dog.  Calm, gentle, willing to play and be trained.  Ideally we need a young dog.  We have started looking around the rescue centres where we live in Merseyside but we can't go on a waiting list so it's about touching lucky.  Any advice or shared experiences would be much appreciated.  Thanks, Tracy