My 2 y/o son has painful large, hard stools about 2x/week. Then he'll have a loose one after that. I give him white grape juice and plenty of liquids.?

Constipation. Your son has constipation. He should be seen by his doctor to rule out physical causes of constipation but in the meanwhile you can start giving him miralax, (polyethylene glycol) an over-the-counter laxative that will help him have softer, more frequent bowel movements. You don't mention his age; his doctor can help you find the right dose.
Diet!!!!!!! This child has alternating diarrhea and constipation, a very unhealthy presentation. Remove all form of sugar, soft drinks, commercial fruit juice, milk (yes----milk) and get a good well rounded multivitamin. Give him enough vitamin c (to diarrhea and then back off) and magnesium from a health food store. This is an all too common story and it is very sad.