How often did your vet suggest giving him the Nemex? I have only seen loose stools with wormers, when there are actually worms present. If the dogs don't have worms, I haven't seen loose stools.
It's not unusual to worm twice, about 10 days apart, and then run another fecal. This should get the existing worms and their offspring.
I have seen roundworms at least, becoming more resistant to the pyrantel pamoate, which is why for adults I prefer something like Panacur.
Here are the CDC recommendations:
http://www.cdc.gov/ncidod/dpd/parasites/ascaris/prevention.htm
and it does say
If the mother did not receive prophylactic treatment, puppies and kittens must be treated early and repeatedly in order to prevent patent infections.22 In areas where both ascarids and hookworms are common, begin treating both puppies and their mothers with an age-appropriate anthelmintic at 2, 4, 6, and 8 weeks of age. Some recommend extending this to 12 weeks and then treating monthly until the pet is 6 months old. To treat for ascarids alone, begin by 2 ½ - 3 weeks and treat every 2 weeks for at least three additional treatments. Because prenatal infection does not occur in kittens, preventive treatment should begin at 3 weeks of age, and be repeated at 5, 7, and 9 weeks. Nursing dogs and queens should be treated concurrently with their offspring because they often develop patent infections along with their young.
Now, don't forget that heartworm preventatives often serve as wormers as well.