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.
'91 Pickup - Imelda / '08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'98 Rhygin Juke SS / '12 Gunnar Rockhound 29 / '16 REEB Dikyelous
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein