My son was 2.6 kgs at the time of birth now he is 5 years and his weight is 16 kgs is it normal???

Maybe: The birth weight would put the kid at 5% on US standards (consistent with the lowest 5% of birthweight) and 10% at age 5.Since growth involves other factors (length, head growth, development) it is hard to say whether this represents a problem or not. If both parents are average for their ethnic group, this may be considered failure to thrive on a medical basis.