I took a pair of jammie pants that fits him now and used that as my guide. I folded the pants in half and using the folded side I cut the fabric about an inch bigger then the actual pant leg to account for seam allowances. At the waist line you want to cut the fabric about 3 inches bigger. I didn't do that and later realized that I should have. Then using the piece that I just cut out as a guide, I cut a second piece on the fold.
I turned one of the pieces right side out then stuck that piece inside of the other piece matching the seams together. That creates a U shape and I sewed the U together. I did a reinforcement stitch on that seam. Now your two pant legs are sewn together, then I finished off the hem on the legs, and put the elastic in at the waist.