My grandson has hit the same stage of growth my son did -- that one where the legs are long and the body is lean. It was horrible finding pants for my son and of course, at that time I didn't even have a sewing machine. However, now I have a sewing machine and loads of time.
These are our test pair. I'll be mailing them to my daughter in law to try on him. However, I think I need to go up one size. The pattern book is one I have had for years and I love. The clothing looks dated, but new fabrics and lets face it -- children's styles just don't change that much. To keep costs low on our sample I went to the thrift store and found two large pair of men's pants. I used the fabric from the legs of a tan pair to make these. (I'm saving the denim for the next size up.) Total cost per pair of pants will be a whopping $3.59. I didn't even buy the thread I used....it was a gift from my husband's great aunt.