Welcome to healthcare Magic!
The drug is a good pain reliever and is not a cause for loose motions.
It contains an opioid
which can rather cause constipation
There may be some other cause of loose motions like some gut infections.
He should ensure hydration by taking copious amounts of liquids like juice, soups etc to match the losses.
If the problem persists more than a week he should visit a clinician and get prescribed appropriate antibiotics according to local pattern of flora.
I hope the advise would be informative and useful for you.