Why on earth does he think US immigration are going to let him in?
The condition on most visas is that you are able to support yourself for the duration of your stay, the US government will have an amount they will expect him to have on him.
Even if he has the money why would he be let in? I assume he has no job to return to or other 'good' reason he has to return such as property or family (as in his children not parents).
I have met up with people I've met online and I have a friend in Canada who has been to see me a couple of times from an online friendship so I'm not totally against this but he does sound naive.
What does the girl do? Is she working? I doubt she can support him for 3 months. What about medical insurance?
Most importantly do her parents know about this? Is he just going to turn up one day? What if they hate each other on sight? what are his plans.
How would you feel about having her to stay for a few weeks?
I agree you need to talk to her and her parents, it could be a lovely friendship that will last for ever, but it could also be so many other things.
Would he be better doing something like Camp America and her do the same?
He also needs to read up on what is / is not acceptable. If (and it's a huge if) it is true love and they decide to marry there and then he could end up in prison for not stating his true intention on his visa.
He could also end up in trouble if he attempts to take Kinder eggs or haggis in to the country. It lots of little things he needs to know.
When I was at uni another student had arranged to meet with someone fro the internet in London. I didn't go to uni until I was in my 30s so I took over as 'mum' and insisted on a phone call at the very least and that I would go with her to meet him. It is what people do, not to spoil things, but because we care about them. He should understand that.
You could stop him going until he is 18, but it might be more sensible to just ensure there are loads of safety nets.