My son was also very underweight in infancy.  We were given a high calorie powder and were told to add a spoonful of it to all his feeds (milk, juice and solids), which is supposedly tasteless but helps children gain weight .  If I remember correctly it was called Duo-cal.  We did later discover however that his fludrocortisone dose was insufficient, which was probably the cause of his difficulty gaining weight - is your child’s renin level OK? - maybe worth checking as after my son’s dose was increased, he did gain weight at a more normal rate.