As long as he gave you the power of attorney to keep it for him, this is considered to be accepting it and you must keep it until he comes. Your zakat was paid on time.

If you pay your zakaat and the person to whom you gave the zakaat, asked if he can leave the zakaat at your house and take it later and the person didn’t come for it after a long time, may be even after two or three months, is your zakaat accepted?