Thank you for your query.
As you have mentioned that your son is passing hard stools it means that he is constipated and that can be the reason for the bad smell. Worm infestation does not give rise to hard and foul smelling stools. There is no need to give Zentel
You need to increase green leafy vegetables in your son's diet so that the stool softens and he passes them more regularly. You may like to visit http://www.vardaan.net/hl-chfd.php where I have provided some instructions on constipation
. In case diet does not help then suppository or enema
may have to be given to facilitate evacuation of the hard stools.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.