You have mentioned that most of the doctors who've seen your child have told as immunity problem. That implies a normal general and systemic examination.
This year, most children have faced the same problem as your child. Possibly because, schools have reopened direct classes after a gap of 2 years and children are exposed to viral infections, facing recurring febrile illnesses,due to their poor immune system.
However time will make it better.
Next Steps
Ensure good healthy diet, supplements with zinc, iron especially to boost immunity.
Lab tests not required immediately ( unless a detailed physical examination suggests any abnormalities or warning signs)