Thank you for your query.
It is about 3 months the lung infection
I am not sure that there is some residual infection.
Chronic infection of any part of the body at this age may create this type of problem.
Such a infection may involve the every organ system of the body.
Try to maintain his nutrition & fluid intake properly.
Neurological problem should be considered also.
also may be responsible for it.
Serum electrolyte test is necessary to exclude potassium imbalance.
If the condition not improving then a neurologist
opinion is required.