Hmmm... I can't think of anything less hard to believe to be honest: this is last minute, peak season, there are literally hundreds of people trying to fly all over the place, and whilst you might have reason to think that the flights are not packed, I'll be a little surprised if you are right.

Incidentally, I think that your new plan is definitely better than the old one but far from fool proof at the moment IMHO. What you have is three separate itineraries, which is a lot more than most people would want here, especially when the third and final one is the one that matters to you most (be it for financial or personal reasons).

Your son now has 3 hours between the two flights and you don't say how long till the flight to Kefalonia (again, I think that hoping for last minute reward space to the Greek islands in the last week of June was as unrealistic as it gets). The two tickets are separate and involve a change of terminal. If all goes well, that's plenty of time, but obviously, it only takes a cancellation of either flight or a bit of weather and the plan could be all in jeopardy so whilst hopefully this will all go smoothly, I'd keep an eye on possible plan B if need be.