What on earth could this be?

[Grublin]

DS1 is 3.5 and has a temp, white ish looking tongue, 'pain' in legs, lethargic but no rash (yet) and very little appetite.
He has a couple of hours where he seems ok (temp down, no pain no lethargy) and plays, then he'll go and sit on the sofa and the temp will rise, the 'pain' will start and he'll be really whiny and off.
This morning he has said his throat hurts.
This started late Saturday night, so hoping to get to the Drs this morning.

Is it going to turn into something like scarlet fever, mumps, chicken pox etc etc?

a virus.

One of those annoying can't be to specific types.

Its awful when they are poorly, hope the gp puts your mind at rest.

Right off to get a bath and get ready to battle to get through to the Drs on the phone!!
I hope it is just a virus that clears up soon!

Sounds like a virus to me too, they quite often involve pains in the legs. Good luck at the docs, have you been giving him painkillers?