I have knitted socks and found that adding a nylon yarn to the other yarn as I knitted the heels kept it from wearing out so fast. You will be knitting with two yarns. Also might help to add it in again when you get to the toe area. After the hole shows up, maybe you could just pickup the stitches from the outside and knit a 'patch' for the heel area and hand stitch it down where it would join the sock.