Most adult dogs only eat twice a day, that's typical. For a pitbull, I would typically feed about 1 cup (340 grams) of dry food mixed with about 2 tablespoons ( roughly 30 grams) of wet food. If I was only feeding wet food, there would be a lot more of it, and if I was only feeding dry food there would be a bit more. Now, if your sister is trying to have him gain weight, the vet may have put him on a special diet that includes more food or more frequent feedings than that. But feeding him roughly that standard for a few days isn't going to starve him.
It is also important to keep in mind that lots of dogs are excellent at acting like they are starving. Dogs who have been underfed in the past, may be afraid that each meal is the last one they will get, so they may constantly look for more food, even if they aren't actually hungry right then. Other dogs have learned that if they look hungry, a human will feed them, so they do it all the time. Basically, you know how much food he ate, and can't necesarily trust him to "tell" you that he's hungry.
If you take all that into consideration and your mom and you both think that he still needs some more food, then there are a few things you can do. Just like if you were cooking for a human, dogs need a balanced diet of meat or fish, starch, and vegetable. My suggestion is 1/2 of a cup of plain cooked rice (measure the rice before it is cooked), 1/2 of a cup of plain cooked lean meat without bones in it- ground beef works well, or chicken breast, or most other meats that you have around. Dogs can eat most kinds of hotdog and processed meats okay, but you should check to make sure there aren't any onions or garlic in them. Fresh, pure meat is better. Make sure that there are no bones in anything that you're feeding him, and cut out any obvious parts that are mostly fat. Then, you'll add a small amount of some raw or lightly steamed vegetable. Most dogs like carrots, if he'll eat spinach, broccoli, etc. those are also excellent.
There are a few "human" foods that dogs cannot eat because they are poisonous:
- chocolate or cocoa
- garlic (a lot of pre-made human food has onion and garlic in it, always check the label to make sure it's safe)
- grapes or raisons
- some artificial sweeteners like xylitol
There are also foods that you should avoid because most dogs can't really digest them and will have diarrhea or an unhappy stomach:
- dairy (including milk and cheese)
- lots of fat
- wheat products, like bread
You should also avoid large amounts of sugar. Just like in humans, sugar is bad for a dog's teeth and stomach.