Why does my child breathe through the mouth?

My son is 5 years old. He used to get cough & cold very frequently and required antibiotics every 15 days. This continued for 2.5 y when he started to wheeze frequently. The doctor put him on a pump and after 6 months he got relief with no need for antibiotics/medicines. After a year, he started mouth breathing during sleep which on investgation revealed large tonsils as the cause. Otherwise he is very active and constantly involved in some activity or other. It is only during sleep that he is in discomfort. Kindly guide.