And yes, I have checked for other signs of meningitis. He can hold his chin on his chest and has been moving his neck around, he has no fever, he has no loose poo or vomiting, he has no rash.
We were out fruit-picking this morning until 11.30 (last halfhour of that he was eating an ice lolly, playing in a sandpit under a canopy, on a bouncy castle (yes, I know) under a canopy, and he was wearing a hat, appropriate clothing, sunscreen. Spent the rest of the day indoors in the cool, doing lego, until 4, when I took him out to the splash park. Sun not as strong and he wasn't wearing a hat 
. It was after 4 by then, but still hot. (We're in SE.)
Slightly picky at teatime but ate everything. Went to bed as normal. Woke, or rather didn't wake, but was crying in his sleep, an hour ago, and gradually became more unsettled until he was responding to us but crying a lot. 
He was hiding his eyes from the light (lamp on the lowest dimmer setting in his bedroom) and eventually demanded we turn it off. NO fever. Gave him some nurofen because he seemed uncomfortable and said he was 'not very well'. He's got a bit of a blocked nose all of a sudden. He was definitely distressed because he wanted to be back asleep, but the light sensitivity thing has me twitching. We've cooled his room down (opened the window) and tried to give him some drink. He's finally gone back to sleep and seems more settled.
WWYD? I've heard that photosensitivity can be a symptom of heat exhaustion but can't find mention of this online. He really doesn't have any critical meningitis symps and has no fever (his skin is cool, normal - not cold/clammy).