Before I start, a disclaimer, as I have no specific knowledge on this subject, but I do have some knowledge on related subjects which I think are relevant. But of course on something that relates to safety, you must be certain of your source of information.
Firstly, I doubt that a CO monitor would register natural gas, as there isnít any (or only a trace of) CO in natural gas. However, I donít know how specific the sensors are Ė maybe there is something in Nat Gas that will set off a CO sensor.
Secondly, the whole point of the smell in gas is that it warns you of the gas even at very low levels of the gas, well below danger levels. It is therefore likely that you can smell gas long before it becomes a danger, and therefore probably before a sensor would be set off Ė nobody wants a sensor to alarm at very low levels as you would get false alarms. I would guess that your sonís sense of smell is likely to be better than a detector. However, lots of things can give off smells, which can build up in an enclosed space, which may then remind him of gas Ė some soft plastic toys such as beach balls, for example. Could it be something like this, with the smell being quickly dispersed when the door is opened and people walk about?
Be an irresponsible eater, have your potatoes chipped