Jeanna,
First off, I'm on your side in this and hate the way some parents refuse to follow through with their emotional and financial obligations to their children. It hurts the kids more than anyone else. However, I am going to say some stuff you may not wanna hear and some practical stuff - in the following catagories: legal, psychological, and from the heart.
Legal: You can consult with a domestic relations lawyer to find out what your rights are, and the court might help you, because the "rights" generally go with what is in the best interest of the child. However, as a divorced woman whose lawyer ex-husband delighted in dragging me through court, I can tell you that escalating a conflict into the courts could tear you and your husband apart. Unless the threat of court scares her into submission, this route could be expensive, time-consuming, extremely emotional and divisive, and it will impact your entire life. You should consider this route fully and carefully before going down it. The expense of an entire wardrobe of clothes won't begin to compare to the time and financial and emotional costs of hiring a lawyer and starting proceedings.
On the other hand, you may want to check out mediation. I did this professionally for a long time (I am also a lawyer), and a good mediator can take very argumentative people from a bad place to a workable solution. They are trained to help everyone get past the angry stuff and get down to the business of problem-solving and child-rearing. It's also much cheaper and faster than court, and it's voluntary, so you don't have to agree to anything that doesn't work for you. In court, on the other hand, you are hoping the judge does what you want him/her to, but you have to live with whatever they say, even if they come up with something else entirely.
Psychological: In my mind you are completely RIGHT about what you are saying, but even so, this situation has a big potential to negatively impact your husband's son or even your marriage. Unfortunately, children take it very personally when parents fight over them. This son might not fully understand it as forcing his mother to do the right thing. He might think you see him as a burden to you and his dad. This is a very bad outcome, and will ultimately undermine everyone's relationship. Think of this situation another way. What if his mother had died? You would gladly take him in, tighten your belt and provide him with whatever he needed. While your actual circumstance may not be FAIR because she didn't die, the message you want to give your husband's son is the same - that you gladly accept him and all that comes with him. I know you DO - but the money fight may cloud his perception. When you marry someone with kids from a prior marriage, it's a package deal. Right now, his son needs your protection and love just as though he really was your son. Where else do you want to spend your money than on your kids?
Personal: From the heart, I'm with you. I can't understand any parent who abandons her children, whether it's emotionally or financially. It's not right, it creates burdens in other households, and the biggest victim is the child, because they DO know when an adult is there for them and when they've been abandoned. Still, I urge you to think hard before escalating this controversy... The costs you're bearing now might be low compared to what can happen if you escalate.