Thanks for your question on Healthcare Magic.
I can understand your concern.
Ideal weight for five month old child is 3.5 to 4 kg.
By his symptoms, possibility of either
lung infection or
bronchitis is more because in both these,
cough with chest congestion are seen.
So better to
consult pediatrician or neonatologist and get done
1. Clinical examination of respiratory system
2. Chest
x ray.
He may need antibiotic, antihistamine and inhaled treatment according to the above mentioned reports.
Don't worry, with appropriate drugs, he will be alright.
Hope I have solved your query. I will be happy to help you further. Wishing good health to your son. Thanks.