It could be a food allergy. What does he eat? Corn is a common allergen for
cats. Wet food is best, since it has more water and less carbs than dry food.

most flea collars aren't effective.  have you checked him for fleas?
I agree with the allergy thought as well, especially if your feeding him
something different than he was used to.

And if your sister in law isn't willing to take him back any time soon, I
would have a serious talk about neutering the cat.. its not fair to you or
to him to leave him intact.

stink up your home with the most ungodly reek you've ever smelled that is
almost impossible to remove.  A mature tomcat is second only to a skunk for
stink.  He also needs Frontline to combat his flea allergies.  Flea collars
and dips don't do diddly.  Take him to the vet and have him neutered and buy
some Frontline and you'll be a much happier caretaker.  If you are caring
for the cat and paying his bills, then you are the de facto owner and need
to act in the animal's best interest.  Otherwise, your sister is the one
that needs to clean up stinky boy cat pee and pay for the Frontline.  There
are cats dieing in shelters by the thousands every single day, and there is
absolutely no reason for this cat to be bred to add to the toll, unless it's
for *greed*, which is the worst reason in the world to breed.  He obviously
isn't being shown, and unless he does get shown and has the points necessary
to prove he is a good example of the breed, you won't get any quality queen
owners interested in mating with him.  "Purebred" doesn't necessarily equate
to a healthy or a good quality animal.  All it means is that the parents are
known.  Do the cat and yourself a favor and neuter it.  It's a favor for
your sister too.  And, it's certainly a favor for the cats in shelters
everywhere.

Sunflower