Cut the seam up one side of the leg. You don't have to go all the way down to the hem, just enough to make the pantleg lay flat so you can sew a patch on. Make a nice patch with edges folded under, sew over hole & restitch the leg seam. All done! You could reinforce your patch on the inside with heavier fabric like denim, or double it, or sew one both to the inside & the outside. I've done this & it does the trick for me!