You don't say how many cogs on the cassette but if he has knee problems there's no reason to stay with an 11 tooth in the rear if a 12 small is available. Even going down to a 50 tooth front gives an almost useless 122 inch high gear.
On an editorial note it's a shame that manufacturers used the advantage of more cogs to extend the upper range of gears instead of offering tighter clusters (meaning smaller jumps between cogs). Stock cassettes beginning with a 12 or 13 tooth small cog are very hard to come by, especially in the better quality level. The 11 tooth cog may be of use with a compact crankset but all it does for road bikes is encourage mashing high gears and the resultant quicker wear, less efficient riding and possible knee problems.
Last edited by cny-bikeman; 02-06-13 at 10:31 AM.