I would definitely go for Whitgift.
Your son's choice is very important here - he is old enough to have a say and clearly loves Whitgift. Bless him for saying that he would understand if you couldn't afford it - he is obviously a lovely boy. But he really, really wants to go, and in fact you could afford it.
I do understand the idea that the money could be saved and used for him later. But is there really going to be anything in the (hypothetical, uncertain) future which he cares about this much and would be of this much benefit? He would still be able to go to the university of his choice, for example; just not with such a big financial cushion.
Whitgift is a truly great school (I am biased because my son goes there). It's especially good if the boy is sporty. It has massive resources compared with state schools - you are paying that money for a reason - and this comes out in the pupil/teacher ratio, the individual attention and nurturing, the specialised staff, the extracurricular clubs and supervised activities, etc etc. A good state school can be very good, but they just don't have the same resources (if only they did!).
I would say that education is about the journey, not just the destination. You have got the opportunity to buy your son a really great quality of life during some of his most formative years.
I would not rely on being able to switch schools in either direction at 13 - there is too much possibility that no place will be available. In particular I think it would be a cruel situation for your son if he went to the comprehensive now on the basis that he could always join Whitgift later - and then found out he couldn't.
This is not a straightforward decision. The money is a factor. The journey time is a factor. The existing friendships are a factor. But I think the biggest factor of all is your son's preference. He has fallen in love with this school and has done extremely well to get through the entrance procedures. My son was in the same position and I could not have brought myself to take that away from him.