Unless he has a huge load of worms that are causing problems I would not be concerned. I am not saying don't treat him for them, I just mean calm down. 
DE (diatomaceous earth) can be used to get rid of both hook and roundworms. It is a slow process though. The Drontal would be faster. Drontal does appear to be relatively safe, so if he is healthy (healthy despite the worms 
) then you could use the Drontal.
I personally would go the DE route because my dogs having a few worms that were not causing problems wouldn't concern me. When I got Addie she had worms but she also had chest and head infections and was underweight so I went the poison/chemical deworming with her to get them out so she could have one less thing for her body to deal with. But in my list of *bad things to use" wormers are lower on the list.
If he tests positive again later on after Drontal then you might want to consider DE added to his food and also put on your lawn to get rid of them there.
He'll be fine.
I WOULD NOT use Diatomaceous earth. There is absolutely no proven effectiveness of this "homeopathic" treatment. As a matter of fact it can severely injure your pet. This is one of those internet things that if you think about it, makes absolutey no sense what so ever on how its supposed to work.
Diatomaceous earth is basically a sand created from the shells of diatoms---small crustateons. These shells can be increadibly sharp, leading to micro lacerations within the GI tract, which if you consider a heavily parasitized animal, can make things much much worse. I've heard two different reports on how this is "Supposed" to work. one is that it literally cuts up the parasites---now think about that you are feeding a substance sharp enough to cut up parasites in the intestine, but not so sharp it won't hurt your dog? Doesn't make sense. The other is it irritates the parasites, causing them to let go and they leave the dog. That falls under the same "UH?" logic. As far as I'm concerned the only thing Diatomaceous earth is good far is certian types of aquarium filters and I'd like to see it removed from the shelves for everything else. Its a waste of money with a potential negative outcome.
Now the question is do you treat or do you not. There are two mindsets with parasites in domestic animals. First is treat everything--these animals are domestic and they do not have a "natural" parasite burden. The second is low levels don't mean much. In all honesty, in an adult dog, if I only saw one roundworm egg on a fecal exam, I'd record it and probably consider asking for a repeat fecal, however it really, really depends on the household. The problem comes with what is "low levels" and the risk of human disease. Just because the dog has it and is fine, doesn't mean that the human won't get it and not have a problem. Thats why many veterinarians push for frequent deworming of animals. To prevent human infection.
Both roundworms (toxacara spp) and Hookworms (ancyclostoma) can cause human disease. Roundworms cause Visceral larval migrans (which can lead to blindness, deafness, brain damage, organ failure etc) and hookworms cause cutaneous larval migrans. This becomes a very, very important consideration with some households because of the humans in that house---ie someone who is immunocompromised or someone who is old or a house with young children---we all know the potential for a child to put dog poop in their mouth---it happens and it happens frequently.
Not only that, hookworms are one of those "perpetuating" parasite infestations. Almost all dogs have roundworms at some point in their lives--the worms are encysted in the mothers body and hormones associatd with pregnancy cause the eggs to be shed out through the milk. This means that virtually all puppies will have roundworms of some species. Hookworms on the other hand are acquired after birth through the contact with contaminated feces or soil with feces containing larva (see cutaneous larval migrans again). Hookworm ova shed in the feces of the dog can then reinfect the dogs increasing their worm burden. This usually cumulates in a dog with a heavy parasite burden who breaks wtih horrific bloody diarrhea secondary to the parasites and their effects on the intestine. I have seen dogs die because of hookworms---severe, severe hookworms.
I don't know the specifics of your dog, but its not unreasonable to consider deworming them and then either entering a check/redeworm schedule or using a once per month heartworm preventative which also prevents round and hook worm infestations. Drontal is a safe drug in my opinion. I've administered it to hundreds and probably thousands of animals and not one time have I ever experienced what I'd consider a drug reaction. Unfortunately, in its safety and effectiveness, its also not that cheap. Just remember, that you can clear the infestation now, but with hook worms, your dogs can easily become reinfested due to yard contamination.
