My 9 yo son has a slight cold and cough, should he avoid soccer practice and match?

Depends : If the kid is fever free and interested in practicing, a cold should not exclude his participation. If he is running fever it is a sign his system cannot control his body temperature at rest, and the extra exertion would add additional stress and risk heat related illness.

Running a fever??: Many athletes perform under mild URI (cold) symptoms and generally do okay. Their endurance and performance may be affected. If no fever is involved and breathing not affected, i'd not discourage his participation. But if he is febrile , having problems with sore throat, coughing a lot, or significant nasal drainage, i'd keep him home and treat the URI symptoms with rest, fluids, tylenol, (acetaminophen) deconges.

Cough: Not necessarily unless he is asthmatic as the asthma symptoms can be much worse during acute infections. If he is eating and drinking nomally and has otherwise maintained the same tolerance for activity then he should be okay. You know him better than others and if your instinct says he should not play then that supersedes the advice of those who don't know him!