I would take the Citalopram in the morning and the Trazodone at night.
Citalopram is for anxiety/depression and Trazadone is to help with sleep.
Also, you have to watch for serotonin syndrome when taking the two medications together. (Source: http://www.drugs.com/drug-interactions/celexa-with-trazodone-679-335-2228-0.html)
Not without consulting with his psychiatrist, as cutting back can cause withdrawal. Trazadone is very sedating, so it's probably the culprit.