My baby will only drink milk

I should not be too worried about trying to 'battle' with your son's own decisions - as far as the choice of fluid is concerned. The amounts that you mention, albeit as milk, are perfectly adequate - and there is, of course, additional fluids contained in his solid food.

You will certainly not be able to exert any major influence on his decisions at this time, and I can only suggest that you keep trying, perhaps with diluted juice at first. Give it to him when he is not necessarily thirsty, for example during a meal. When he is ready to accept a new taste experience, it will happen. Good luck and don't worry!