My son is 10 years, he has a severe pollen allergy. I gave him zyrtec at 5:00pm, can I give Benadryl (diphenhydramine) at 12:00am?

Agree with Dr Novick. Are the tree pollens starting up already where you live? If the combo cetirizine and Benadryl (diphenhydramine) aren't working consider starting a steroid nasal spray (quite safe actually) or could he be coming down with a cold?
Pollen allergy. You can but you can give him another dose of zyrtec (cetirizine) in the morning if necessamry.
Mgt. I would suggest avoiding pollen, by closing the windows, using a HEPA filter indoors. Nasal steroids and saline rinses may also help. I would using Zyrtec or Claritin.