There may be something lacking in his diet, mineral-wise, or he just might like eating dirt. I know plenty of dogs on excellent food and supplements who still like to eat dirt. Some heartworm preventatives also protect against other types of worms, so if he is on one of those, that will help. Just keep an eye on him and get stool checks twice a year.
What kind of food is he eating?
What kind of food is he eating?