Hi Janel, You are correct in assuming that the Malleoloc Ankle Brace will help with subluxation of the peroneal tendon. Check out this article about just that: https://www.myfootshop.com/detail.asp?Condition=Peroneal%20Tendon%20Subluxation. It comes in only 2 widths: Wide and Normal. The measurement to figure out is the width of the heel. For the Normal size, your son's heel width in inches needs to be less than 2 3/8 inches. I imagine it is possible for his heel to be too small for a proper fit. It might help to refer to the manufacturer's site for sizing and then give them a call: http://shop.bauerfeindusa.com/cgi-bin/cart.pl?Dynamic+products.txt+MALLEOLOC+template.html+3