Boarding Suggestions for 6th Form Boy

Gordonstoun -very international, full boarding, accessible via Inverness or Aberdeen airports. Sporty, lovely countryside setting, co-ed and happy!

Monmouth School for Boys. Strong sports, mix of boarding and day. Lovely setting. Transport to airports.

Wellington students go to organised parties in London at weekends, about to get a new head.

That should read St Pauls

What about looking at a school that offers IB eg Sevenoaks or Wellington College.
If he is very academic Westminster and St Oauls may be considered but as London Schools the boarding will be very different to a country boarding school.
Another idea would be Atlantic College but admissions may be closed but it is truly international.
If they don’t want standard British Public school another option maybe St Chris in Letchworth. However the cohort is more mixed ability. Not London area but I believe Shrewsbury is full boarding

When are you looking for? Most first choice schools will already have allocated y12 places for new entrants for September 2019. There may be some movement post GCSE if students fail to make grades but she may wish to ensure her DS is on the waiting list.
If parents are abroad you want full boarding which these days seems to mean the big all boys schools which are unlikely to have any free places, Oundle, King’s Canterbury, Sherborne (if you want single sex)....St Edwards Oxford ( though that is now taking weekly boarders), Marlborough, Uppingham, Rugby and I think that is about it.....
I would not look at Whitgift if I were overseas as it is predominantly day. If you want IB you might look at Sevenoaks but it is very competitive. All UK boarding schools have an international vibe as the British can no longer afford them.
Depending on where the boy wants to go to University, state boarding for sixth form might also be an option. Schools will have fewer boarders, but most will be full boarders. Many European families use them. Peter Symonds in Winchester for example sends more to Oxbridge than many of the big name public schools for a fraction of the cost. Students also then tick the state educated box.

Another for St Edwards Oxford (Teddies). I believe it offers all sports he's interested in. Very down to earth and known for it's pastoral care and support. Staff are very supportive and more than happy to give extra help if students require. Lots of international students. There's a coach to assist with transport to London for school holidays/leave weekends. They're allowed into Oxford on a Saturday night but have to be back quite early. If they stay onsite, there's a cafe/bar and sometimes shows/entertainment, don't know if all houses do it but DD's HM gets them together for a chat/drink as well.

St Edwards Oxford? Ambitious head so the bright do well especially those joining in the 6th form quite a lot of international children liberal loads of sport including an on site golf course generally very happy children mainly full boarding and good for airports.

Reeds! seriously? It's a day school, with some weekly boarders but very few proper boarders. And it is not very academic, the average A level point score last year was 38.34 and a grade B, some of the local state comps have better results. Your friends DS would be left on his own in Oxshott Woods every weekend miles from anything.

This is why education consultants are worth their money. They will find your friend a proper boarding school to suit his requirements.

Whitgift or Epsom? A levels or IB?

Pretty good I think, not sure how they compare to other boarding sixth forms. Your friend would need to check it out if she's interested. It's definitely well-located for airports.

Reed's School in Surrey? Not sure whereabouts your friend is looking, but Reed's is academically good and very sporty, definitely offers the sports you mentioned to a high standard. Quite a few international students too.