Hi I am new to Pet Forums

I have recentely found a stray male cat who is 2 to 3 years old. He had been hanging around the local supermarket for a while and every time I saw him he had lost weight. I brought him home a few days ago and tried to feed him but he wouldn't eat and hardly touched his water I tried to give him dry and wet cat food, chicken and tuna but he didn't want. I managed to give him a few treats and today he had a bit of my milkshake (not ideal) and some more treats then he was sick. He has only used his litter tray to wee. Apart from all this he looks healthy apart from being underweight, he has no fleas, good teeth, clean ears, no blood in his urine and no swollen tummy. He is breathing fine and doesn't have a tempreture. His third eye is showing slightly though but there's no discharge. I should mention I have three other cats who have left him along apart from the odd peak round the door to which he hisses and they leave. My cats are all neutured females and as far as I can tell he is neutured, his testicals are not swollon. Is he just nervous or should I be worried?