Its not as likely to be a preditor as you think. If he’s smart, he already knows she’s real. I went thought this with my family and it damaged our relationship. Let him go, but keep in contact.
Not a very good idea to go alone
You mean should you let a young adult live his life? Yes, yes you should.
I mean, you can say no all you want, but he is 18
Be careful. Even girls
Lure boys for sex trafficking.
Young one, people will say “he’s 18 , an adult, his own man let him have fun” - NO seriously with times as they are. You can’t stop him, but maybe he should provide more information and/ or talk with the young lady. - Kind of scary for you - but actually he owes it to you - out of respect and love- you’ve been together-tight - since he was 12 - you two need to have a 1on 1 talk it out.
You might have him watch “ web of lies” series on ID channel (
) it has stories similar to what your feel and dealing with. 
Go on the trip with him, but let him set up meeting the girl on his own
I’d ask to meet her on FaceTime make sure she’s who she says she is,just for your peace of mind and her safety.Reality is tho he’s 18 and can do whatever he wants if he has the money