S.D.
I would probably try and fight it. Especially since they didn't give you all the information needed to make an education decision. It's $25 which hopefully to them isn't worth the bad publicity they'd get otherwise. If they don't budge at least it's only $25. If you do end up paying it, tell them that you'll have to set up payments.
On a side note, do you have your son take Lysine every day? It's a vitamin. I've suffered from canker sores in my mouth since I was about his age. Stress brings them on really badly for me. I find that if I take a 1000 mg of lysine every day that it keeps them in check. It's really inexpensive at places like Walmart. You may want to give that a try and see if it helps him. To give his body a jump start, you can even have him chew them up and swallow the first few times so it will absorb faster. It's not the best tasting but tolerable. Good luck!