Hi Ingird
Sorry to hear about your poor little one.
There must be a bug "doing the rounds."
A very good friend of my son's (ten yrs old) has suddenly come down with this and looks like he'll be missing his sport this weekend, which I always think is the sure sign they are really ill!
As with all of these bugs, as long as the children are big/old enough and kept warm and full of liquid it tends to go away. The only time I would worry is when they are really small and you feel they might get overwhelmed. I couldn't see any mention of how old your son is?
As always, if in doubt, go to the doctor, that what we pay taxes for!
Good luck and do tell us how you're getting on.
A