My baby has been sick for weeks and is losing weight doctors not helping?

[Skye]

My son is nearly 18 months and was 2 weeks ago 8.4kg now sine being sick is only 8kg i have just put him onto pedisure and he is slowly taking it, but refuses to eat now he has a runny nose and is very lethargic i am going back to the pediatrician tomorrow but does anyone know anything i can try for rapid weight gain for a baby