I think I would AT LEAST call your families doctor.
Motrin is not the same as Tylenol. The body can actually manage a lot more tylenol than the dosage allows for. It isn't the same with Motrin. Tylenol is acetaminophen, Motrin is ibuprofen. Acetaminophen can cause liver damage. Ibuprofen doesn't cause liver damage, but can cause stomach or intestinal damage. I'm not sure though if it is something that is more of a long term use issue or not.
Was the liquid a children's forumla or adult formula?
I'd call your doc, and if you can't get him, I'd consider a trip to the ER. Did they even tell you any symptoms to be watching for with your grandson?
Here is a list of symptoms of overdose I found on "drugs.com":
"Overdose symptoms may include nausea, vomiting, stomach pain, drowsiness, black or bloody stools, coughing up blood, shallow breathing, fainting, or coma."
I don't think I would just do nothing...