The only people that can be married in Westminster Abbey are members of the Royal Family, Order of the Bath members and their children, and anyone living in the Abbey's Precincts.
St Pauls ...
http://www.stpauls.co...sic/Weddings-Baptisms
St Paul's appears to be out then !
Wonder how far out the precincts of Westminster Abbey stretch out and whether anyone lives within them now. When I got married in Llandaff Cathedral (Cardiff) I found that there was a flourishing business among residents in the precincts. For a suitable fee ('rent '?) they would allow the bride-to-be to declare herself resident with them . They also required that she lodge a suitcase of clothes with them. This was duly done.
You never know. Unlikely, but that might be possible with Westminster.
have a look at their website, there certainly are weddings in Westminster Cathedral, but it would (I believe) be a Catholic ceremony with Mass, as it's a RC Cathedral?
Can your arrange for your grandson (or one of his parents) to get on the New Year's Honours List? If so you'll be in time to arrange his marriage in St Paul's Cathedral (but, even then, only in the crypt):
http://www.stpauls.co...sic/Weddings-Baptisms