Tell him you don't want to replace it.
Wait until summer to decide whether its worth it to run the fan all the time. You may find that the house that has been fine without a furnace fan since 1947 will continue to be fine without a fan running. I mean, this is purely speculation, but if it was unbearable, someone in the last 70 years might have put a fan switch in. If it is indeed unbearable, you can easily DIY a thermostat. The instructions are in the package it comes with. At worst, you'll have to run a 4-lead wire from the furnace to the thermostat.
Running the fan will circulate the air better, but may not appreciably contribute to comfort. If you have a dehumidifier on your furnace, you will need to run the fan.
Also, the extra wear from running the fan all the time will mean your furnace fan will need replacing sooner.