Fantastic trip - except for the end bit.
Sounds like DS may have been suffering dehydration, ratherthan flu ?
In hot weather you need to carefully check how much people, esp kids drink. They get excited and forget to drink enough. We had a bad experiance with DD not drinking enough. If you think back, was he drinking and weeing enough ?
I was thinking that too, Aussie but he drank at least 2L of Tang, was peeing every 45 mins or so and we took a break every 20 mins or so to drink in the shade. He seemed fine until, pow, it hit. He had stomach cramping and, shall we say, severe digestive upset.
He did have to be reminded several times to out his hat back on though...