An extension can be the Call Cover for a line. #206-7-{Line Number}-3, #208-{Line Number}-{Covering Extension Number}
Allow the covering extension to use the Remote Call Forward feature #322-{Extension Number}-1
Put his cell phone number in Personal Speed Dial 99 on the Covering Extension, and program a button for Feature-11-{Extension Number}-99
Press the Call Forward button before leaving in the evening, and incoming calls will grab a 2nd line, call the cell number in 99, and conference the lines together

If it is only calls TO his extension, say from the night auto attendant, then just leave the lines as they are, give him Remote Cal Forward, program the speed dial and forward button, and then the forward is on, calls transferred from the AA to his extension will forward to his cell, all other calls get normal treatment