My impression from Nico, and also Daphne du Maurier (I just uploaded a couple of her letters to me re Michael and possible suicide which she didn't believe btw*) was that Michael was frailer than his brothers as a child - see also Sylvia's Will, where she says (from memory) something to the effect that "I don't want Michael to be pressed too much as at present he's not very strong" which Peter (in his Morgue comments) thought might be a reference to suspected TB - but as Nico often reminded me, he was "fit as a fiddle" at Eton, excelling in just about every sport, except of course swimming ...
* Search for "Daphne Andrew" - more to follow ...