this may happen
In between when there is so e viral or bacterial infection this can happen as also when there is some allergy
So give symptomatic treatment like anti histamine
syrup or cough syrup or nasal decongestant drops prescribed by your pediatrician
as and when necessary.
Obviously for 2 months he was better according to you.
Give some more time for growth.
can be tolerated hoping that he will improve by time.
I feel at least till 5 years you should wait for surgery for adenoids.
Humidify the room ,use otrivin
pediatric nose drops at night
Do not worry.