My 19-y.o. stepdaughter has been in my life since she was 8, living with her mom most of the time. We went through the same scenario -- on visitation weekends, she would come dressed in threadbare clothes, or outfits that were more appropriate for a preschooler than a girl about to enter puberty. We bought lots of clothes for her in those years, so I can relate.
Not knowing the personalities and issues involved, I can only guess at what might be at play here. One is that your husband's ex might be trying to stretch that child support dollar, knowing that if she sends the kid over to dad's wearing worn out clothes that don't fit, you'll take him shopping. The other is that the kid has picked up on this -- he is 11, after all and no dummy, and knows that if he wears the nastiest stuff he's got when he goes to dad's, dad will buy him the cool stuff that mom won't buy.
Either way, you've gotten sucked into it, but it's not too hard to get out. He's old enough to have a mature discussion about the dynamics of your blended family. He's old enough to understand fairness and sensitivity to others' feelings. No accusations, no nasty comments about the ex, just an honest discussion about how things should work. Even though the child support check should cover his needs, there will be times when it's appropriate for you to jump in and do something a little extra. You need to find that good place between being the non-custodial dad who's nothing more than a child support check and the disneyland dad, who goes all out to entertain on visitation weekends.
Making blended families work is hard, even under the best of circumstances. My two boys, ages 8 and 4 love their older sister dearly, and we explained to her early on that just because she doesn't live with us full-time, she is not a guest in our house, she is part of our family.
Good luck...hope this helps.