Do i need to look after my cat differently, i think he was the Runt you see?

Hi, I have a cat that was the runt of the litter, he was 10 weeks old when we got him, and he just fitted on the palm of my hand. I think when this happens you do tend to treat them different to the other cat's, i don't mean you favour him but he gets that little bit extra. My cat is like yours one minute he is friendly the next he does not want to know. So i just leave him now to get on with it, cat's are cat's and only do what they want to do and when they want to do it. Charlie my cat is 10 years old now he is still very thin and now he has a bad heart, but i know he is happy and i know i am doing everything i possibly can to help him with his problem. But he still thinks he is special little boy against my other cat even though i show them both the same love and affection.