I'd be having nightmares if this was my son.
Does he inject himself daily? Can he take all his meds with him via US border control? Will he need a doctor's note to take them? Is it possible that they might confiscate his syringes/drugs, thinking he might be a druggie?
He obviously needs GOOD MEDICAL INSURANCE for 90 days in the US, (which will be expensive I think, if he discloses that he has a pre-existing conditrion ie diabetes which he must do to get the cover), and what if this girls (and her Mexican family) feed him only junk food so he ends up in a coma?
What if his host(s) are criminals and want to use him for some reason? (Eg Harvest his organs or get him to carry drugs back to the UK?)
I know I am getting carried away in thinking the worst, but this is what would be going through my mind. I am not being helpful, but wanted to say that YANBU to be worried.
Hope you can talk him out of going or find a way to stop him. Failing that, can you go with him for a week?
Agree with posters who think he might not get through the US immigration with his story of staying for 90 days with someone he has never met. (And this would be my hope if he actually boarded a flight.)
I understand that you really want this trip to be a good experience for him, (which it could still turn out to be), but what if it's the worst experience EVER and you didn't try to stop him going?
At least if he waits till he's older, you will have done something positive, as he will be that little bit older, and he might start to see the potential dangers?
I believe that young ppl, (esp males), have a different, (ie weaker), way to assess risk, until they reach the age of about 25 ... which has some evoutionary advantages I suppose, (eg they are willing try things which might come in useful), but there's a certain % of them who die or have lifelong problems as a result of being (too) impulsive and not understanding all the possible risks. And, although he is 17, it doesn't mean he's able to cope like a adult with a lot more life experience.
Really feel for you OP. Hope you can look into this girl and her family more if you are unable to stop or dissuade him from going.
Let us know if he goes? (Maybe someone on here will be able to help him, if things start to unravel once he gets there?)