Stall forcing a decision for as long as you can! Emphasize that there is no pressure - that this is an amazing opportunity but that you love him, he can have an amazing HS experience at home, and you will 100% support him whatever he decides (and mean it). Tell him that you appreciate that, like a cat who pauses in the doorway before going through to take a reccy, with a big step like this it is entirely natural to take a pause- especially when he hasn’t even seen the place yet!
He’s what-13/14?! This is a huge world change, and the difference between an idea and reality is meaningful. Take the pressure down as far as you possibly can. If money has come up, make it clear that the money is not the meaningful metric- yes, it cost something to do the testing, apply, go visit- but that is the sunk cost of creating the opportunity and doing the due diligence, and a ‘no’ answer is as acceptable to you as a ‘yes’ answer.
Then see how the visit goes. Watch him, what face he shows the people at Prep and what he says just to you. Weigh that against what you know of your boy and how he typically reacts to new people & places. Consider his weaknesses, and try to see both where the pressure points will be and how they might be amelioraed. Consider his strengths, and try to see how they will be encouraged and supported.
When you get home, go for a tour of the HS he would attend if he didn’t go to Prep- even if he knows it/has been there before, take the tour as a prospective student.
Then listen.
ps, if he does go, you will have to take out an insurance policy to cover the tuition if he withdraws- typically he has to have attended classes for 2 weeks or similar before it kicks in for a non-medical reason.