Thank you for your query.
Mucus discharge from the nose is no reason for not vaccinating your child. There can be a number of reasons for mucus discharge. However, it it is greenish in colour then secondary infection has taken place for which appropriate antibiotic needs to be given.
If the doctors say that the discharge is due to allergy
then you need to discuss with the doctor so that the substance causing the allergic reaction
can be found out and tried to eliminate from your son's environment, if possible.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.
Rajiv K Khandelwal