How To Relieve The Congestion Of My Kid S Nose?

My kid s nose is congested almost all the time. he cannot have quality sleep at night. it started when he was 2 now he is 7. i used saline in the mornings to clear up when he was 2. it stopped working after a year. last year i start using otrivin pediatric at nights, but not consecutive days. 3 days at most then i did not used it for 2 weeks. sudafed does not work. doctor prescribed Desloratadin but it works only till 4am. i tried humidifiers, mentol mist nothing works.i used otrivin during my pregnancy aalmost every night at 3rd trimester but after the birth i did not need it anymore. how can I make him breathe through his nose? it affects his sleep, his speech, his reading, his teeth, and his throat