I'm sorry that you and your son are going through this. My husband was similar to this when he was 20 and taking sertraline. Your story sounds very familiar, right up to the ambulance situation too.
Have there been any other behaviour changes that you have noticed?
I ask because my DH showed the same behaviours as your son, he was in and out of the GP on various different types anti depressants for years and seemed to get worse, including being suicidal and making some very scary attempts to commit suicide.
It has taken us 17 years , but at 37 he was diagnosed with Bipolar 1. The "depression " was being misdiagnosed due to the suicide attempts. When in fact, it was mania. The antidepressant medication was contributing to an even higher state of mania. He actually needed mood stabilisers.
He has finally been prescribed lithium, which he takes alongside orforil and quetiapine. Quetiapine is an excellent mood stabiliser which has a "side effect" of acting as a sleeping aid. The psychiatrist couldn't emphasise the importance of stable sleep enough.
Your son is at the age where bipolar can start to become apparent and it so often gets treated as depression at first, which can cause symptoms to worsen. One of the reasons why the medication may have worked at first but not now could be that his mood cycle has shifted or is shifting into mania.
I'm not a doctor. I saw so many similarities in your post that I wanted to share our experience as I wish someone had done for us. We've been together since we were 15 (I'm 37 now) and this "journey" has taken nearly 20 years.
Feel free to pm me for support and really push the doctors to consider different medications. Sending you and your son lots of love.