Don't worry about it! Physiologically/developmentally speaking, it's better for him to nurse from the breast than drink from a cup. For my 14 month old, we keep a little sippy cup (with a flexible straw) of water around within his reach, and he drinks from it throughout the day, mostly during meals. There are times that he'll refuse the breast but prefer the water instead. He wants to drink from our water glasses sometimes too, and we just hold the cup for him. Just follow his lead; there's no need or reason for him to be drinking from a cup.