How to add weight onto a slim child without harming his health?

Go easy here. Please be happy if your child is healthy. I am concerned about your worry. Children differ and fluctuate throughout development. A good chat with your pediatrician will reassure you. If this just part of ongoing or many concerns, let's take care of you first.
Review your child's. velocity of growth in height, weight & head circumference as well as BMI with your pediatrician. If the growth trajectories are normal & BMI is 5th-less than 85th %ile, there is no need to worry. Healthy kids eat what they need when nutritious food is presented at pleasant, sit-down family meals during which parents model good food choices & good manners. .
Diet for slim kid. Kids eat rather by instinct and would not be able to be hungry, as they do not have a big reserve of calories as we do. If they receive plenty of love, if they are playful but not hyper, if they sleep well, and poop daily, then they eat enough! we can always invite them to eat some more to satisfy our worries. If kids grow up without love or face family stresses, they would eat ++ to compensate.