I think your health worries are making you spiral and his behaviour is having more of an impact.. and you are working yourself up which can't be good for you. I'm sorry you are going through this worry and I hope that you get the results you hope for.
It troubles me to read him described as "trouble since conception." (how could you possibly know that?) and "a thrill seeker" who is always in A & E. "He is the problematic third child" like he's the difficult third album or something. You say that he doesn't know about these labels but I guarantee you, he absolutely does, it leaps out of your posts and it will certainly come across in your attitude to him. It may make him feel like the family scapegoat who can do no right and that can really affect him, so that he thinks Oh well if they think that's what I'm like, why bother? I may as well live up to it.
He's home safe. That is the main thing.
He's now a sixth former and a third sibling, so he's used to seeing older siblings out partying. He was with friends, not alone. He's still finding his feet at sixth form and making new friends, he will soon find out if they are a bit over the top. It's a learning curve for both of you.
I don't think "punishments" work that well and are more stressful for you. I think he's lied to you because he knows what the reaction would be if he told you the truth.
Try to talk to him (preferably not at home but maybe a cafe or with pizza or similar bribe so he knows this discussion will be different. ) Sit him down adult to nearly adult ie not as an angry parent who will kick off if he says the wrong thing (tempting though that may be) Tell him you are feeling under the weather and this has made you extra worried.
Reassure him that he can tell you frankly why he was late and ask him WHY he thinks its OK for him to lie about it because you need to be able to trust him to give him more leeway. And ask him how he would feel if someone did that to him.
I think you would get a lot further with him with this approach and its worth a try given the alternative. Ask him what he thinks is fair in terms of going out and get him to say why... there must be some meeting ground there. You want to create an atmosphere where he is willing to talk to you about things like this, because then you can give advice.
Its different for everyone but The rules we had were,
- no big outings on a school night
- nothing that would affect getting homework done.
- always go with friends (preferably ones we knew)
- prove they knew the routes home.. and how and where to get the night bus if they miss the last train connection.
- Carry spare cash for emergencies, and a charged phone which is on our find my phone.
- text if there is any change of plan and keep in touch.
- Also share the number of one or two close friends (having sworn first I'd never use it)
- And if they got into any kind of trouble, they could ring and we'd help.
We still had our ups and downs but these rules generally worked. And its better that they do this learning curve now than when they are far from home on their travels or at uni etc.
Maybe get your siblings to take him out and show him the ropes, (or judge if they think he's capable of handling it) That may go a long way towards reassuring you, so that you can concentrate on your health and feel less anxious.