Different kids have different metabolic needs, and no matter what publication you read, it won't be tailor made for your son. For instance, the Gerber people recommend a huge amount of solids for babies, and neither of my boys EVER ate as much in a sitting as they recommended....and both of them are healthy, and growing like little weeds.
Where do I fit in a snack? There just doesn't seem to be time. Maybe his meals are too big? DS was underweight for a while so when we started solids, we were urged to feed him until he was full and he eats quite a lot at mealtime.