I'm afraid I agree with sparkle. Water dragons can vary quite a lot in both appearance and personality.
How old is your son?
What if you find one, spend the £100 odd it'll probably cost.. then he notices, or the water dragon doesn't like him, lunges for him, bites him, is skittish, because it's going to be scared and confused.... then you'll have an animal he won't like because he'll know you lied to him and replaced it.
And if your water dragon was not appearing old and you don't know why they died, putting another one straight into the old enclosure could result in another sick water dragon. Best to remove everything, bleach the viv, let it dry and make sure there is nothing that could be passed on to the new animal.
It's thursday.. which means no TNT will be sent out really till Monday now.. so even if you find one on here, unless you can collect it locally, the very earliest you could get it would be Tuesday.
I would just be honest, kids have to learn about death one way or another.