Zyrtec, claritin, singular, and allegra don't work on my 7 yr olds allergies at night (nasal drip & cough) only benedryl. Can he take it every night?

Dose? Zyrtec & Benadryl (diphenhydramine) are very similar. He should get similar relief if he's taking the right dose for his weight. Benadryl (diphenhydramine) will make him more drowsy. You should talk to his doctor about an rx nose spray to control allergies like Flonase or nasonex.