With my daughter, it was a texture thing. She didn't care for the textures of certain foods and would avoid them.
Right now, the biggest thing is to introduce a variety of things so he can try them. As long as he is still drinking his milk, whether that is breast milk or formula, and is alert and active, I wouldn't worry too much.
One thing we did that helped make the transition to baby cereal easier was to mix it with breast milk instead of water. We also made it really thin at first, like just a touch thicker than breast milk. That way it tasted familiar and wasn't too different of a texture. As she got more used to it, we started to thicken it up.
If your son likes fruit juices, you could also try making the cereal with juice.