Hi,
Weight gain may not only be a function of parasitic infestation. It may be a good idea to have his stool/feces examined for the presence of worm eggs. If there are indeed worms, then treatment with medications to eradicate the worms should work.
General advice would include hygienic considerations: proper handwashing and monitoring places where he would be playing. Keeping a diary of things that is actually eaten is a bit tedious, but it may help point out some reasons for malnutrition. Some candies like bubble gum and chewing gum, may seem pretty innocuous, but there are reports of weight loss associated with abnormal absorption of food in the intestines.
Blessings to you and your child.
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