This doesn’t sound like a typical presentation of selective mutism. Selective mutism typically presents when a child is in preschool and it would be very rare for it to suddenly occur at a later age. A diagnosis also depends on whether you see a difference in communication between situations where he is relaxed, such as with family at home, and social situations – such as school or with strangers at the shops or other activities. If there is less communication in social situations then this could possibly be social anxiety disorder or selective mutism. In any case, I would recommend that you have your son assessed by a psychologist or neurologist to start to investigate the nature of the difficulties communicating and from there treatment can be planned.