There are different schools of thought about how fast is too fast, although I think pretty much everyone is in agreement that you don't want them to grow super fast. I followed the slow growth plan, which has a recommendation that a puppy gain no more than an average of 2 pounds a week. Here is a link from my puppy's breeder's website about the slow growth plan.
http://www.jrsgoldenangels.com/slowgrow.html
That said, obviously not everyone subscribes to this philosophy and it is on the low end of ideas about acceptable growth rates. It's something that I believe in, but it is not the only approach. Puppies can tend to have growth spurts, so I wouldn't worry about it too much unless this is the norm. 5 pounds a week is a lot. Also, people are used to seeing puppies that are overweight, so even if he doesn't look overweight to you, he might be. You should be able to feel his ribs very easily.
http://www.jrsgoldenangels.com/slowgrow.html
That said, obviously not everyone subscribes to this philosophy and it is on the low end of ideas about acceptable growth rates. It's something that I believe in, but it is not the only approach. Puppies can tend to have growth spurts, so I wouldn't worry about it too much unless this is the norm. 5 pounds a week is a lot. Also, people are used to seeing puppies that are overweight, so even if he doesn't look overweight to you, he might be. You should be able to feel his ribs very easily.