If he were my dog I would have him checked again. I test a few times a year regularly. However, I would test at any time if there were symptoms.
Hopefully, someone will pop in with more knowledge, but I believe detecting parasites also depends on what life stage they are in when you test for them; sometimes they aren’t detected. It’s been a few years since I have had a puppy and I have never had one with a serious parasite problem.
Something else to consider is something in his environment, cleaners, laundry soap, lawn fertilizer, is causing these issues. It could also be a protein common in both of these foods that are causing him issues. Proteins found in meat, grains, vegetables, fruits etc.
Healthy table food is fine for dogs as long as it is no more than 10% of his daily caloric intake; you want to keep his meals balanced.
