Redcloud, it's more of the fact that Glenn as far as we know never had a relationship with his biological father since his real dad abandoned his family, so it's very unlikely he would use that name. I'm pretty sure his step-father actually legally adopted him so Glenn's legal last name is Frey. I agree too that he probably was cremated since there's no burial information out there and even though people are famous burial sites are usually easy to find with how the US keeps those records open to the public (same with birth, death, and marriage certificates and divorces).