You need to just go by his body condition IMO. You should be able to easily feel his ribs but not see them.
It also depends on the food. Honestly Beneful is one of the worst foods out there, if you can afford something better I would definitely suggest it. You'll find with a higher quality food you don't need to feed as much because it isn't full of fillers like the cheaper foods, he should poop less too. I would also suggest going to an adult or all life stages formula.
You can take a look at these sites to get an idea of the different types of food. The reviews are not written by a nutritionist but they are still a good guideline IMO.
Ehh, that seems a bit much. My adult corgi gets one cup a day, and he's 30 pounds. I would say no more than a 1 1/2 cups. I also agree that you should try to find a better food. Even Nutro Natural Choice is a pretty good food, with a lot fillers. You'll notice that your dog will poop less, and it'll be easier to pick up. :)
Also, don't pay attention to the feeding guide on the back of the food. They always tell you to feed more. That way, you run out of food faster and buy more!
My pup is on Beneful as well (for now, because it's what the breeder was feeding her...) and she gets 1 1/2 cups per day at 3 months old. Her weight gain and growth have been completely normal so, for her, this is enough. You should go by how he looks and feels, as well as his growth.
I don't have any experience with Puppy Pro Plan, but I am slowly moving my pup over to Taste of the Wild. It's a high quality food and, best of all, it's an All Life Stages food. So no need to buy a separate puppy formula!