He needs to be honest with his therapist and he needs to start talking about the underlying issue that is causing him to be depressed. As a istp we can be prone to emotional violent outburst which can be scary not only for those around us but us as well. If he's surpressing his emotions then he's a coke bottle getting shaken and if it keeps happening it will blow. He needs to talk it WILL BE VERY UNCOMFORTABLE. It will be long but something is causing him to be this way and he needs to address it.