Did you mean Improved Natural Attack?
Yes, but he has to be Wildshaped into an animal when he learns the feat...
Just kidding.
All feats are available to everyone, even if the feat is found in a Bestiary or seems to only apply to monsters.
However, there is a bit of gray area here. The prerequisite says that you must have a natural attack to take this feat. Humans and other humanoids normally do not (fists, kicks, headbutts, etc., are "unarmed strikes" not "natural attacks" for the purposes of game rules). Which means that the druid in question would need to be an unusual race or else he doesn't have a natural attack.
A strict DM could rule that a human (e.g.) doesn't have a "natural attack" so he doesn't meet the prerequisites. And he would be within the RAW to make that ruling. A less strict DM would rule that sometimes you do have a natural attack (when using Wildshape) so you can qualify.
Is that second ruling legitimate by RAW? Maybe. I have a precedent but it's the Fly skill (not a feat):
"You cannot take this skill without a natural means of flight or gliding. Creatures can also take ranks in Fly if they possess a reliable means of flying every day (either through a spell or other magical manner, such as a druid’s wild shape ability)."
Based on this, a human with no natural flight cannot take ranks in the skill until he learns a spell that lets him fly, at which point he can. I think this sets a rules precedent that can be applied to feats too, so a human with no natural attacks cannot take Improved Natural Attack until he learns a spell (or Wildshape, etc.) that gives him one, at which point he can.
Either way, this is a gray area and DMs might rule it either way, so make sure you check with your DM.