Sadly a problem saying S or Z is often ridiculed.
Brian, aged 16, referred himself. He was taller than me. He wanted to join the police. He said that when he was under stress some of his S sounds would come out a bit like TH. He was afraid that this might happen if he was making an arrest. He didn’t want to be laughed at by whoever he was arresting.
I asked if he would mind if I tried to listen to his problem, putting him under a bit of pressure in the clinic. He said: Fine. So I asked him to talk to the wall. And sure enough I soon heard exactly what he was describing.
With his agreement I gradually put him under increasing amounts of pressure. And he kept a diary, listing every time an S or Z sounded wrong. Six weeks later he was standing on the chair shouting at the wall while I pretended to ignore him, noisily shuffling papers. Under this artificial stress from my rudeness, there were no more instances of a wrong S or Z. And there were no entries in his diary. It seemed that his problem was resolved.