DS3 weird illness, fine in day but goes downhill later?

[AFingerofFudge]

Not quite sure how to word that title!
Would love a bit of help please working out what is wrong with DS3 who is 9. It's not something I've come across before.
Last Wednesday night he unusually woke in the night and complained of a headache, but by the morning seemed fine again. He went to school Thursday and was fine but by the evening he was flagging and had a temperature. This has been the pattern since Thursday. He gets up and is his usual self but then by the afternoon he goes downhill. Low grade temperatures, complaining of a sore throat, lethargic and slightly off his food ie he'll eat it with no great enthusiasm and doesn't want pudding (v unusual not to ask!) Also a bit of a cough but nothing major.
I had to collect him early from school this afternoon as he was flagging and yet this morning I had absolutely no reason to keep him home as he was behaving as normal.
So I'm totally puzzled and wondering if anyone else has come across this??

Anyone?

Sounds like he’s fighting off something viral. If it’s been going on since Thursday, I’d keep him off tomorrow and try to get him seen by a GP. Make sure he’s having plenty of fluids as well

Thanks for the reply. Yes I am making him drink lots of water as I wondered at first if he'd had a bit of sunstroke. But now I think it must be more than that as it's so erratic.

When I was like that it was Glandular fever.

Any insect bites recently? My son had erratic symptoms like this but including muscle aches after an insect bite. Turns out the insect bite was a tick bite and he'd contracted lyme disease. After 2 weeks a bullseye rash appeared on his back (but a rash doesn't always appear). He was treated with antibiotics and made a full recovery. If there's any chance your DS may have been bitten by a tick (they are tiny and can be hard to spot) best get him checked out by a GP asap.

We had something like this one Christmas, it was miserable. It was viral, I think & as it was Christmas, we were just able to hold up until it passed. Miserable to have at this time of year though.

Thanks, I shall look for any marks, but to my knowledge he hasn't been bitten.
And yes, it probably is just one of those viruses but I much prefer things that fit into some kind of box when it comes to illnesses, I don't like non specific!