Please don't dose your cat without the consent of your vet. If your kitty is getting sicker or having difficulty breathing, take him back to the doctor, or call and see if this medicine is appropriate.
If you give him the wrong dose, or give it for the wrong amount of time, or if the medicine is too old, you can actually make the illness worse, or harder to treat, by building up a resistance to the meds.
Most importantly - is your kitten eating and drinking? If he is not, then he needs to be hospitalized as kittens who become anorexic can dehydrate and die rather rapidly.