Hi My son is 5y 9m, allergic to dustmites has asthma which we are treating with foxair & Venteze. Visit pediatrician every 2 months, since he was born. He also had his adenoids @ 4. Every morning I notice his nose is blocked full of phlegm.

Please start over. You get the most from this site when you provide background information on a problem & then ask a clear related question. No question was asked. Your posts are not linked & go to the site at random.
Not 100% treated. It sounds as if your son is not adequately treated if he awakens with phlegm. In the US we have antihistamines and leukotriene inhibitors in dissolveable pill form that could benefit your son. .
Mgt. I would suggest a virtual consultation, but based on the information that you provided, some suggestions include nasal saline, nasal steroid spray and avoiding dust, smoke, mold and pets. Does he have any skin conditions such as eczema or food allergies?
Allergic Rhinitis . With his history of Asthma (Atopy) it is likely that he has allergic rhinitis and his nasal congestion is being triggered by allergens. Suggest you use an appropriate nasal spray like Avamys. Add a non sedating antihistamine when necessary (desloratadine). ENT appraisal if persists.