I would not cut into the sock. I would add a line of zig zag stitching and leave the extra fabric where it won't wear on the leg. If he wears them under slacks it won't matter. If he wants to wear them with shorts, put it on the inside of the leg. The reason I wouldn't cut into them is you are going to get a run and there is a lot of tension on the seam and a regular sewn seam I don't think will hold.