We chose DGS for several reasons, a major one being the IB (others included languages specialism, diverse learning community, and really liking what we saw on open mornings and evenings)
Ds is strongest in subjects that suit the IB approach IMHO - valid points made here about some university science/medical courses preferring A levels in the right subjects - MFL, arts and humanities. My view is that the IB's learning and teaching style can be a better preparation for independent critical thought, text analysis, and context understanding when studying some subjects at degree level, than the equivalent A levels in their current incarnation.
Overall it's not necessarily better, just different, and its differences suit ds even now in Y10. There is a lot of talk about how a very high points score = five A* A levels - but in all honesty I'm not bothered about that aspect of it.
I believe that DGS is also the only English state school currently following the IB Middle Years Programme, which means that boys already at the school have a good understanding of the IB methodology before they enter 6th form. It's also the largest IB state school in the UK, and regularly gets some students in the top scoring cohort across Europe.
So whether you feel the IB is the right approach for your son, DGS do know what they're doing with it!