When you feed him meals, offer him water or juice in a glass, but don't worry about pushing formula. When you go back to work, you may find that you want to be able to breast feed him at bedtime or first thing in the a.m., and your milk supply will adjust to that after a few days. and remember that he's MUCH moee apt to take a bottle from anyone other than mom, whom he knows has the good stuff. You could try leaving him with a family member and seeing if he'll cry it out and finally accept a bottle. But, really he doesn't need the nourishment so much now that he's getting food, and he will adapt.
I would just offer the glass, and let him learn to drink from a real glass, or cup, that you hold for him. At some point, he'll have fun feeding himself, and having a drink while eating his meal, may be something he likes. :-)
But don't panic. All things in good time. And in the meantime, enjoy your son. He'll grow up so fast, and it's wonderful to simply relax and enjoy the babyhood moments.