Yes, you can assume his old call sign assuming he has passed away or stopped renewing it and no one else has claimed it. Check the FCC website under the vanity call sign program. You as a relative do have a special access to the call sign (if for example you present a death certificate) but after a period of time it would become available to anyone.
"Stand your ground. Don't fire unless fired upon, but if they mean to have a war, let it begin here." -- Capt. John Parker