Nazly, worth remembering what a fever physically means. A raised temperature is a biological response of your body fighting germs. So it means your DSs body is doing exactly what it should be doing - fighting and destroying a virus.
Your DSs temperature is not a sign of a virus, it is a sign of his body fighting the virus. So actually the temperature is a good sign, not something to be immediately worries about.
Assuming his temperature doesn't go so high to suggest something else is wrong, then there is no need to treat an average temperature with antibiotics.
The reason NHS say 5 days is because after 5 days his own antibodies should have been able to fight the germs, which is much better in the long term. If after 5 days his body is still having to work hard to defeat the virus (so still has same raised temperature), then he might benefit from antibiotics to help fight the virus, hence seeing the doctor then.
Your DSs immunity will be higher if he is left to produce antibodies to fight infection naturally. This will mean tolerating a high temprature and treating as a symptom (fluids, cool clothing, calpol as needed). That way he will stand a much better chance of not getting ill next time he is exposed to the same virus. So he won't be poorly so often.