I need help writing a letter to inform someone that they being down size from a full time employment to part time and when thing get better they will be promoted back as full time.

I would not tell them they will go back latter, since "latter" you do not know for sure what you want to do and don't want to be forced to do something that you may not want to at the time.

Next why a letter, why not call him into the office and tell him and give him a copy of the payroll notice changing his status.