How does one weigh a person in a wheelchair?

my 26 yr old son is a quad and is losing weight rapidly. we are having a hard time finding a way to weigh him. he's too heavy for me to lift. his dr advised us to go to a meat dept in our local grocery store. i was mortified that he would even suggest such a thing. i asked his staff if they could look into it and they really were no help at all. any suggestions? he weighs under 150 lbs at last check so he's not obese. also, uses a power wheelchair.