My pit bull has sometimes not ate but would go back to eating that night or the next day so at first I didn't think nothing of it but he hasn't been eating his food for a couple of days now but will eat a little of human food. He is still drinking water but he is a little aggressive towards the other dog. If I put his food down he shows interest but won't eat but is very protective over it. Can Anybody help me. He is 3 years old

Is your dog intact or has he been neutered? If he is intact, do you know if there might be any females in your immediate area who are in season?

Are there any changes in his environment such as new animal, person, furniture, etc. Has anyone or pet left the household?

Is he acting normal other than the food issues? Is he playing, sharing toys, using the bathroom, etc. Is he showing any signs of not being well such as not wanting to be touched in certain areas, drinking a lot more than normal, lethargic, anything that is out of the ordinary for him?

Before giving him human food, there are other tricks you can try for getting him interested in his food. One is slightly heating the food to release its odor. Another is pretending to eat it yourself. The danger with giving him human food is that he may learn that ignoring his food gets him your food. Our food isn't always good for dogs and can lead to health problems for them.

If he is showing aggression over his food, can you feed him in a separate place than other animals. Don't allow him to 'guard' his bowl. If he isn't eating take the bowl away and try again later.

If you are concerned he may have gotten into something or is beginning to be ill, don't hesitate to contact your vet. It is always better to be safe than sorry.

Are you feeding on a schedule or free feed? Feeding a certain amount at certain times is recommended over just letting them have at it. It is easier to keep track of, and will keep the other dog from getting his nose bit for being too close to this ones food.
Just to echo Cat here, neutered or intact? And, how is he otherwise?
Human food is not recommended for pets as it can cause some pretty serious problems, if all else fails,
1-heating it a little
2-trying different foods
3-leave it down and give him plenty of time to eat then take it away.
Then, you can try boiled chicken and rice or boiled hamburger and rice.
Make sure you drain the fat off, a dogs body cannot process the fat in foods like humans can.
A healthy dog will not starve himself, he will eat.
But if he is "off" and not acting himself, you would be best advised to book him in to see the vet.