Sticky situation.....We bill out cost of wormer plus $2 for dosing it. However, we lay that out in the contract. I think you have a right to be upset - especially because it seems like he's just going for a cash grab.
My best advice to you is to pay what he asks now, and try to work this out with him for the future, however unpleasant the conversation may be. Try hard to be business-like and keep it to the facts. Explain your concerns and try to keep a good open conversation. If you can not reach a reasonable conclusion or an argument breaks out, you should move your horses. I know you said boarding is hard to find where you are however, you don't seem happy with the owner in other aspects as well. Perhaps you should begin your search for a new place now just in case you reach a crisis point and have to move quickly.
SS you are dealing with this - keep us posted as to how it turns out.