We have a 4 and a half month old female kitten and last week brought home an 8 week old male kitten for a playmate.
At first the older kitten was hissing and very upset but that stopped after the first couple of days. When we leave them together now they begin to 'play fight' but if left for more than a few minutes it starts to turn really rough. The older kitten pounces on the younger one and bites him and he starts crying out. We are scared to get angry at the older kitten because we think it might make matters worse. We've been told that they just need time and they will learn to get along but it doesn't seem to be getting any better. Its more like the longer we leave them together the worse it becomes. Also my older kitten has stopped eating.
Anyone had a similar problem and know what's best to do?

First off... cats play rough. Really rough. So what you are seeing may be nothing more than play. Are they hissing, spitting, growling, and making other noises? Are their ears flat against their heads? Are the fights slow with lots of posturing or really fast attacks? All of those are signs of aggression as opposed to play.

Just as an example, I had two kittens about the same ages as your two, and the older one had the younger one pinned down, viciously biting her belly while the little one clawed madly at the older one's head, squalling piteously. I was about to break them up when the little one suddenly noticed that the there was a ball within reach above her head, and she stopped attacking the older cat in favor of batting at the ball! All while the older one was still "attacking" her. That's when I realized that this looked a lot worse to me than it was to them.

This is a pretty detailed description of cat communication, which may help you figure out if you have an issue with your two, or if they are just playing.

CAT COMMUNICATION