He will drink more if he is thirsty. If he still refuses to drink, try offering him some water that is already warm or cold. If he does drink, try adding a little more water to the bath. You can also use your arm and gently shake him so that it's easier for him to chug those fluids.
If he still refuses to drink, you can't force him. There are a few other methods that might work for getting your dragon hydrated though.