What Causes Appetite Loss In A Toddler?

My son 2.3yrs. He is not eating any thing close to 3-4 month his milk drinking habbites Also decrasing. He drink milk in a day 400ml only. His weight and growth not incrasing from last 8-9 month. We went to couple of doctor they give supplementary syrup but no help to all. We did blood cell test also only weight and hemoglobin came less. Not days he is looks like tired and irrtited. I am thinking to give him zentel for worms. Please let me know do to identify he is having worms. Any other suggestion?