First of all, shouldn't your Son be doing his geography homework, rather than getting you to do it?
The reason that it is colder the higher you go is because of how the air is warmed. Solar radiation penetrates the atmosphere and heats the earth's surface. The earth then re-radiates this energy back, heating the air above it.
Because warmer air is less dense than cold air, it will slowly start to rise. However, as it gets further from the earths surface, it will gradually start to cool, until it reaches a temperature similar to the air around it, and its ascent is halted.
The best way to imagine this is a hot air balloon - the air inside the balloon is heated, and it rises, even if the burner is switched off. However, sooner or later the air inside the envelope will have cooled with nothing to heat it, and the balloon will stop climbing.
The exception to this rule is an anabatic wind. As the sun heats the side of a mountain during the day, the warm air mass in contact with the side of the mountain will warm, decreasing its density and causing it to flow up the slope, meaning that the temperature could increase as height increases - however, this is only a very localised effect due to gravity restricting the ability of the air mass to flow up the mountainside.
I hope this helps.
Neil
