I second expat and everything she has said, she is the voice of reason and experience on this thread and I urge you to take her advice. I cannot wrap my head round the number of people saying 'stop babying him, let him go, I/my child/ my yoga teacher's cousin's best friend went travelling at 17 and we were fine, it was a brilliant experience' as if the fact that they were alright means he will be too.
What's the worst that could happen? Texas is another planet compared to Scoland. Probably the worst thing that could feasibly happen to him in Scotland is either being bottled or arrested. In Texas I cannot even think of the worst that could happen to him. It is an entirely different mentality over there, people can and do carry guns, and shoot first and think later. The police are not like ours, who generally want what's best for you and the public and try to avoid drama where possible. In the states (and of all the states it had to be Texas) they are far more aggressive. They won't take pity in him, should he end up in trouble, they won't patch him up and send him on his way. They will see him as a scrounging criminal immigrant if he ends up in trouble. He will not be able to be independent from the people/person he plans to stay with. They will know this. He apparently does not. Unless he has a huge money safety net he can only get out of there with help from them.
He is being secretive-why? Because he knows there is something to hide. If it was all above board he would be excited, telling you all about her and what's going to happen, what her place is like and her family. Give you the address, the dates, can you take me to the airport? He knows something is wrong but he wants to do it so he's ignoring that and not telling you the whole story. You cannot trust any of the information you have, you evidently can't trust him, if you speak to her you can't trust her either and I'm sorry to say it but raising a child to 19 doesn't guarantee you are a good person. You can't trust her parents either. I would be expecting the parents to be equally eager to contact me and my child directly and have the same concerns as I would have. They have not contacted the OP, even just to ask about allergies or the like.
That's just a few of the red flags that occurred to me while reading this thread! Over my dead body would either my child or I be getting on that plane. Because that is the reality, it doesn't take a huge stretch of the imagination for it to actually be his life on the line. He is not a confident, experienced traveller who knows how to get himself out in an emergency, that is not the character we've been told about at all. He is a shy, lonely, vulnerable boy.
Please OP come back and update us, I may be being melodramatic but I am concerned for both of you.