Hi Paul, and welcome to astronomy. It looks like you did a lot of good forward planning there. I can see you're hooked. Lets hope your 12 year old stays interested. A Dob is a great starting scope as each session you will spend less time setting up and more time using it. Very portable as well, so you can put it in the car and take it anywhere. It's great that both of you understand and have accepted that you're not going to see all the wonderful colour enhanced images that you see on the www and in books, and it hasn't deterred you. All those years growing up looking at the saucepan, and then you finally get to see M42 in it's glorious detail. How awesome is that.
There's a few astronomy tools that will make viewing more of a pleasure and less of a struggle.
- Download a planetarium software. There are some great free ones on the web. I use Stellarium. http://www.stellarium.org/. It's also available for android & iphone, so you can take it anywhere and wow your friends with your newfound hobby.
- Use a planning tool so you can just go out and find stuff you want to look at. There are online & offline ones. Astoplanner is a good offline one, & I use Tonight's Sky as well. http://tonightssky.com/MainPage.php
- This telescope simulator is good as well for getting a feel of what things may look like. http://www.12dstring.me.uk/fov.htm
- IceInSpace forum is amazing. I've been interested in astronomy since I was just a young lad - about 40 years. Since I've been coming on here I've learned so much more than I wold have from books. Other people's experiences are a great teacher.
Most of all, use the scope as much as you can. It's important that your son sees a galaxy pretty soon so his enthusiasm doesn't wain, so I got a list of galaxies off Tonight's sky for you. It's for tonight, from 9PM for 2 hours. Then he can go to bed and not be too tired for school tomorrow. Yay... Have a crack at M74, to the west. It should appear something like the image I've attached (this is simulated through a 6 inch relfector, like your Dob, & 13mm eyepiece)
Happy viewing.