Have you taken him to a child psych? My daughter was very shy from a young age and despite every parent telling me their child went through it and grew out of it, she had selective mutism which has required alot of psychology sessions to deal with it.Luckily we got onto it early when she was 3, we didnt want her starting school and feeling anxious, so maybe you could get an assessment from one to see if there is anything you can do to help, if you havent already?