There are lots of tutorials online (including this nice one on pockets: http://www.21wale.com/2012/01/do-it-yourself-cargo-pockets.html?m=1 ). One tutorial in particular was most helpful. I recommend this one: http://shwinandshwin.com/2012/03/boys-cargo-pants-sew-along-day-1.html
So I used what I learned with the last pants for him, plus looking at his cargo pants and shorts and my husband's.
First step is to trace some pants or use another pattern. I tried to trace but it's super hard with all the pockets and adjustable waist. I have 2 new uncut boy's patterns but neither looked good and both looked about the same as the pants from the same pattern I've been using for Ms 3's skirts and jumper. So I started with that.
Most everything else is eyeballed. I hope he will wear them.