15 year old son becoming more violent

my son is really struggling with his emotions at the moment the only emotion he shows is anger

he gets extremley violent when having a melt down and we as parents always gets the brunt of it but tonight

he attacked his school taxi driver in an un provoked attack whilst driving .

i just really wish we could try and get to the route of whats going on as we are really struggling at the moment .he aslo has speech problems so he can struggle to get people to understand him which is understandbly frustrating .any advice would be fab ,is there any kind of apps out there that he could possibly use to help him express himself ?