40 degrees in a little baby or an adult is quite different to in a toddler. In modern houses, wrapped up children with central heating who are incubating a cold can easily get to 39/40 degrees and seem fine.
My general rule is that I treat the child and not the temperature. So, if the child is uncomfortable or in pain then I medicate but I don't medicate purely for a temperature ie I think if I didn't know she had a temp would I even worry? A temperature is the body's way of fighting infection and I don't like to reduce it unnecessarily. This, of course, does not apply to a baby where all temps must be looked at. If my dd has a temp I strip her off, keep her cool and offer lots of drinks.
If the temp responds well to medicine then this is a very, very positive sign, especially coupled with him seeming well in himself. Temps that respond to medication are generally caused by a virus.
You are his Mum though so if at all concerned then see your GP or phone NHS Direct.