He retains grab because the template says it does:
It retains any extraordinary special qualities that improve its melee or ranged attacks.
This is a case of a more specific ruling taking precedence. This exception includes grab, poison, rend, rake, etc. If you turn a tiger into a skeleton, you will have a skeleton with a bite attack with grab, two claws, pounce and rake.
Other special attacks that are not extraordinary and that improve it's attack, like Energy Drain (Su) or a disease of supernatural origin, are simply lost when the template is applied. Or even special abilities that are not related to attacks, like Fear Aura or Gaze attacks.