I believe it is 5% of their body weight at first and then they can go gradually go up to 25-30%. I think I read they shouldn't carry more than 40% of their total body weight. When I started Willow I just started her with it empty and then gradually started adding really light things. Then eventually a small water bottle. In never really got much heavier than that with her. Now, when she wears it she takes, plastic bags, keys and cell phone. Yeah, you should definately start off very light and work your way up otherwise he might get sore or injured.
The other thing to keep in mind is to make sure the pack is placed properly on the dogs back. I see a lot of people with the backpack up too high on the dogs shoulders. It should be centered so it's pretty much right center of the back, not up high closer to the neck. If you try some on him first it would help because you'll see some brands just don't fit some dogs properly.
Willow's is a wolfpack, it's www.wolfpacks.com. If you click on their products, she's got the saddle bag one which is not very bulky. It's nice for if you're just going for a walk or a hike and not camping or something like that. It's also very easy to put on and off and stays in place with velcro.