Desperately need help, please.

I am a married autistic adult we have a teen son with autism. My son’s needs are very complex but the we are have massive issues with his non-stop self talk. From the moment he wakes until he goes to sleep he doesn’t stop talking. Alas this isn’t a two way conversational talk it is an unrelated stream of echolalia. After each random quote he will often demand we repeat what he has says, and should we not reply as quickly as he would like he will get angry and often swear and become aggressive. He can’t pause his speech even when eating which is causing issues with digestion. Has anyone experienced similar issues with a third party or child/teen? We have tried numerous health professionals and no-one can seem to identify the issue let alone help us help him overcome it. We are deeply concerned that this is effecting his social development beyond measure. 
thank you for your time and we look forward to hearing from you.