It's my understanding you can go before a judge once a year to modify child support orders. Of course, our last dealings with child support was in the late 90s so things could have changed since then. Technically, your ex is right in that the child support he's ordered to pay includes all expenses for your son. He is not REQUIRED to pay for those additional things as well as the monthly support. However, if he does, in fact, make more now than he did when your original child support orders were decided, you could be entitled to more money from him to support your son.
You have a couple of choices. Before you file for more support, you may want to approach your ex and ask for additional help with the things you need, like new school clothes, school supplies, birthday parties, etc., and give him the opportunity to help without having to modify the orders. This could save you the expense of a lawyer and him the expense of additional monthly funds coming from his pay. I would only do that though IF you have an amicable relationship with your ex. If he knows that you need more money and you are willing to go back to court to ask for more, he may be more willing to help outside of the usual monthly support. When my husband paid support to his ex, he would often buy new school clothes, school supplies and pay for half the birthday party expenses and other expenses so his ex wouldn't go to court and ask for more money from him. It worked out for both of them in the long run to work out those issues between themselves. She wasn't asking for anything unreasonable and my husband was happy to provide items he knew were going directly to his son.
On the other hand, your ex may resent you asking for more or "threatening" to go to court for more and look for ways to sabotage your efforts to get more support. Some men quit their jobs or find ways to "hide" some of their income. Only you can guess what your ex would be more apt to do given these scenarios.
If you're all set to get more support, I'm sure there are people who can give you referrals for family attorneys in your area. We used an attorney in Fort Worth for our last custody change so I don't know anyone personally to refer you to in Lewisville.
Good luck!
