If they have not tested him for exposure to rodent bait that might be it.
You can ask the vet about that.
As your cat goes out if he ate a poisoned rodent this could cause a problem.
You can read about rodent bait here
http://marvistavet.com/html/rat_poison.html
Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care
One would be a clotting test.
Another possible issue would be a bone marrow problem where the cat's body is not making blood cells at the needed rate. Your vet would call that situation a nonregenerative anemia and they would see that there are no or very few immature RBC's in the sample.