Evening and night-time itching around the bottom in a toddler is classic pinworm (threadworm), and a year since last deworming is more than enough time for reinfection. At 11 kg he needs albendazole 200 mg single dose (that is 10 ml of the 200 mg/5 ml syrup), repeated once after 2 weeks to clear the eggs that hatch later. Treat the whole household on the same day, trim his nails, hot-wash bedding and underwear, and wash hands after every toilet trip. The poor weight gain often improves once the worm load is cleared, but if he stays underweight after deworming he needs a proper growth and diet review.
Next Steps
Give albendazole 200 mg now and repeat after 2 weeks, deworm the whole family together, and book a consult with me on Practo or at Synergy Multispeciality Hospital, Miraj if the itching persists or his weight does not pick up.