As you will learn if you ask his doctor, ABSOLUTELY NOT. The barest minimum age for feeding an infant anything other than breast milk or formula is four months. Preferably, you should wait till he's six months old.
If you have started this practice, put it to an immediate end. His digestive system is not able to properly handle anything but breast milk or formula and his health will suffer more and more with every feeding of cereal or anything else that is not - again ONLY BREAST MILK OR FORMULA. By far of course the best thing for him is breast milk, so by all means, keep it up!
I am a little concerned by your saying that your baby "is always hungry after nursing." He should just nurse until he is not hungry anymore. So if he stops nursing on his own, assuming he isn't overtired and is drifting off before he is truly full, then he shouldn't be hungry after nursing. Don't place any limits on how long he feeds or when.
L.