My suggestion, put up poster, "CAT FOUND" (descriptions, but without photos) on TNB pillars around your neighbourhood, but do not mention anything about the cat having a collar, when someone calls, test them (just to make sure that they are the owners!)

If no one comes forward, then, you'd probably need to find a home for him (me! me!) or if your sis is willing....

Good luck and love to meow meow!