Can you give a 15-year-old zyrtec and benadryl (diphenhydramine) d at the same time... he weighs hundred pounds?

No: Zyrtec and Benadryl (diphenhydramine) are both antihistamines which should not be given simultaneously because the total dose will be too high. You can add a decongestant that does not contain any antihistamine. With allergies that are so severe that one antihistamine is not adequate, the next step is to add a nasal steroid spray. This can be prescribed by your MD. Good allergan avoidance is also important.