My cat came last night with some blood stains on his jaw and head, apparently been injured during a cat fight. Since then he has not eaten and just curled at his bed. I'm wondering what is the worst case scenario? How likely is it that he's got FIV from the wound?
I am sorry to hear that your cat is not well.
If your cat is not eating for an extended time, take him to the vet.
Also if the wounds look serious do the same.
FIV is mostly transmitted by deep bite wounds, which allows the infected cats saliva to enter the tissue of the injured cat. Wikipedia - FIV
Generally cats that can go outside and especially males can get into territorial fights with other cats. Some slight scratch wounds are normal from my experience and heal without a problem.
Your cat may just be tired from the fight. If he keeps up not eating and seems sluggish to you, take him to the vet.
If you are worried about FIV in general you can consult your vet since there is a vaccine but you should know about the effects before you vaccinate your cat.
Please let us know how he is doing and what course of action you took.