You didn't tell us WHICH iMac he has.
If it's one of the new 24" iMacs with USBc ports on the back, be aware that most card readers have USBa type connections and a standard USBa cable won't work.
You'll need something like this:
The Cable Matters USB 3.1 Type-C to Type-A adapter provides a convenient connection between the newest devices with USB Type-C and legacy USB peripherals with Type A. It adapts the Type-C port on a computer to connect all legacy equipment, peripherals, cables, hubs and chargers that ...
www.amazon.com
Plug the card reader into the adapter cable, and then plug the adapter cable into the USBc port on the Mac.
If the Mac has USBa ports (2020 iMacs and earlier), this won't be needed.
EDIT:
There's still a problem in that the adapter above is "on the short side". He would have to be reaching around to the back to find the port every time he wants to use it.
So, I suggest one more cable "in the mix":
Color:USB3.0 Extension 3ft <>NC XQIN Super Speed USB 3.0 Type A Male to Female Extension Cable Color:black Cable Length: 3 Feet /1 meter Transfer Rate: 5 Gbps USB 3.0 High Speed Extender Cable and Usage This cable lets you extend USB connections by 10 feet between your computer or Mac an...
www.amazon.com
Set it up for him this way:
USBc-USBa adapter --> USBa extension cable --> USB card reader
Now, the 3 foot extension brings the card reader toward the front of the Mac, must easier for an older person to "get to". He'll like this arrangement.