I have a 19yr old DD planning on going travelling this year. I am really anxious about it, but am helping support her wish so that I know she is doing everything properly, and she knows I am supporting her and is therefore fully open with me and sharing it all with me.
She has been planning and dreaming of this for two years.
She has been saving hard - indeed I am shocked how much, and very proud at how much.
She has filled out all the relevant forms for vaccinations and the GP is aware and advising her.
She has been to the bank and has a credit card set up.
She has been to several events that are held around in our town for teenagers wishing to travel and see the world, and has had conversations with sponsors ready to help support her and help her find work while she travels. On that end, she knows exactly what visas she will need - and we are going to pay for those for her.
We have internet banking set so that 'should' the worst happen, we can send her money, altho I actually don;t think that will be needed, she is that prepared.
She has looked into the customs and ways for the places she wants to go.
One of the countries she is planning actually has an old college friend of mine, she is aware, and will be acting as parent for me - and my DD will be staying with her for some of her trip.
I would MUCH prefer for her to wait another 6 mths until she is 20, as in my head, I think she will be treated a lot differently to being a teenager, even tho age is really just a number.
My main concern is that altho she is street smart for the town we live in, she isn;t for the countries she is going to - it is all hugely different - but I am really looking forward to how grown up and changed she will be when she returns. She will leave me as my child, and return to me as a much wiser woman. Yes, still my child, but with experiences and knowledge that even I don;t have - what an adventure. I am actually a little envious.
Oh, another thing - we will have savings and back up for very worst case scenario and me needing to fly out to her.
And where is she going that she needs to do all that, and I need for her to be ready and wise - New Zealand and Australia ! Hardly scary, and in fact very similar to the UK in some ways, altho much much bigger and hotter and far away.
Your son has done none of that it sounds like, bar getting something like a visa. He doesn;t have back up funds, he is a loner and doesn;t sound 'street smart', and he has just £350 - I would struggle to survive a week in London on that, let alone going halfway round the world, to somewhere that Expat has explained to you is massively different to here.
And still you seem fit to accuse them of 'getting their knickers in a twist' ! 
You stated in your OP that you are going out of your mind with worry. Believe me, none of what you say makes me believe that for an instance. I sobbed on my GP just this week due to being so anxious about her going - I do have form for overthinking things. At no point will I refuse to allow my girl to go tho - she is doing everything properly, is determined, and she has planned everything she is able to plan, perfectly. We have everything in place set up if an emergency happens, and still I am scared for her - but that is ME, not her, and I will be waving her off with a smile knowing that by doing that, I know she will come home and not rebel and run away.
Your son is acting like a loved up teenager acting on a whim, and you are foolish if you think this is going to end well. I pray it does, but your attitude to people offering real and valid advice is to be rude and, in fact, act like a teenager having a tantrum!