For backpacking I have gone the ultralight route and picked up the Gossamer Gear 2012 Mariposa backpack. What led me to this pack is that it has a large volume for an ultralight pack and most importantly, the hip belt slides through a sleeve so that it can be replaced with a Think Tank Pro Speed Belt. (I also attached some Velcro to the belt so that it attaches better to the Velcro inside the sleeve of the pack) The Pro Speed Belt is padded and comfortable and it allows you to use the Think Tank holsters, including their lighter weight Skin series holsters and pouches, to hold your camera and accessories. The light weight pack and my other light weight items help offset the weight of the camera. While you will spend money on backpacking gear, you could outfit yourself and your boy with a lot of ultralight camping gear for much less than your $1,500.00 budget. The only other issue with this approach is whether a Mariposa is large enough for the both of you. Whether it is big enough may depend on where you are going. If it is cold you will need a lot more clothes and heavier sleeping bags.
One significant caveat to this approach is that all you may really need is a very small inexpensive portable camera, with video and extra batteries, that you can stick in your shirt pocket and pull out at a moment’s notice. As an older guy, I have come to realize that the countryside is likely to be there for a long time, but the experience of traveling with your family is fleeting. My suggestion is to not worry too much about capturing the surroundings in all the glory your dslr can provide, but to focus on getting a bunch of shots of your son and the both of you together. I am willing to bet that 15 years from now you would much rather have a lot of shots of your son than the landscape. A compact camera of a lesser quality than your dslr will likely meet these needs just as well and may be more readily available to capture the moment.
You may be an experienced backpacker so this will not be news to you, but if you are not, don’t weigh yourself down too much. You don’t want to feel like a pack mule and not enjoy these fleeting moments. However, if you are willing to accept the weight of your dslr, the Think Tank holster system allows you to access your camera almost as quick as pulling out a compact camera. Plus, you don't have to accept the limitations of a compact.
There is also a whole forum section devoted to backpacking with kids which can be found at http://forums.backpacker.com/cgi-bin/forums/ikonboard.cgi?act=SF;f=993107219
Have a great trip.