I understand your concerns, but it seems sad that this modern world can often put an 'unhealthy' slant onto things, that in all probability, are entirely innocent.
Perhaps, consider the personality, academic ability, and range of interests of each of the boys. Is there some common link the older boys might share with your DS?
Also, how large is the school? Very small schools tend to have much closer contact with the various Year groups, almost to the extent that in the playground, there hardly seems to be a range of ages.
But I also accept that 'unhealthy' things can, in very exceptional circumstances, reveal themselves, with possibly unfortunate results, and very, very rarely tragic outcomes.
I would be interested to hear more about the the school, and the 'character' of the boys.