One of the most difficult concepts in soccer is the offside law. By the way, it is offside (singular) not offsides (plural) like in American football. Also, soccer has laws, not rules.

It is important that the players, and the parents, understand this important idea. A very thorough animated presentation of the offside rule:

http://www.dynamic-thought.com/OffsideClickette.html

Please enjoy it with your son.