How long should it take my son to advance through each rank? How long before he makes Eagle Scout?

To answer your second question first, your son may not make Eagle Scout. Not all boys do. In fact there are statistics that show less than 2% of all boys who join Scouting will achieve the rank of Eagle Scout. As to how long it takes to advance through each rank. That varies by individual. Each Scout will advance at his own pace. Some Scouts are more driven than others and advance faster. The import thing is that your son keeps completing his requirements and gets them signed-off by the Scout Master, an Assistant Scout Master or one of the Eagle Scouts.