Keep doing what you are doing. By the extras when he comes for long visits and pull of the tags so she can not return them. If she wont get on the phone, then no money, ignore. If son says he needs something, ask him what size or send you a link online from a store, and buy it, send in mail.
For other things, pay directly to the place/activity/dr office. Send her a letter/email stating that if there is something he needs to pay for, to let him know hte telephone number/address and he will pay his half directly to them. We have dealt with dentist, braces-ortho-dentist, bowling ally, dance classes. Most are happy to take your check in the mail for your half and some made our own account.
You did school shopping, the next time he comes (THanksgiving or Christmas?) if you want, you can take him shopping again. Then maybe again for spring break.
If she takes you back for more
child support, then the extra clothing stops. Child support DOES cover clothes, shelter, and food. Look at his order and see what the
court order says about extras. If it does not say anything, then he is not required to send anything else.