You should get a more clear explanation from him than guessing yes, especially since he prescribed drugs for you. You should look up to see of the parasites he gave you drugs for cause the symptoms you have.
You also should really know how many calories your getting instead of guessing that you eat a lot of food because you might be doing yourself a disservice if it's not enough. It will be easy to see if the amount you eat is what is actually causing you to lose weight.
Seems to me if you have parasites and they are causing you malnutrition because they are feeding off your food, it would be such a large infestation that you'd have very visible parasites in your stools when they die off and any doctor would be able to spot it in you.
Cover your bases with the simple explanations before you go off and start thinking things that are more complicated.