i dont know if this will help but you could give it a try
very very dark brown sugar in boiled water might help to get him moving my son had problems going some times not for 3 or 4 days and it helped him. sounds like your son has had such a hard time poor little chap i think you are doing everything you can and it must be such a worry for you maybe he needs to talk to some one to help him get over his fear.