is not a medicine to gain weight. but its not wrong to prescribe it once every 6 months to one year in a child as worm infestation is fairly common in children.
the dose is generally 200mg once only at bedtime in children below 2 years (syrup is preferred as tablets need to be chewed before swallowing or can be given to small children after crushing mixed with water.
regarding weight, a weight of 8.5 kg at 13 months of age is low but not malnutrition
and the best way of increasing the weight is to give nutritious high energy and protein diet to a child.
as long as the child is behaving normally, is active, achieving milestones normally, there should not be any worry about weight at this moment.
wish your child good health