The other moms' suggestions are great.
Try adding chocolate to the milk-- remember that email we got from mamapedia yesterday that said the unhealthiness of chocolate is worth the tradeoff of the healthiness of milk?
Try feeding him from a normal adult cup; my son (14m) loves that. He still can't figure out a straw.
Try different nibs on sippy cups. Some are easier than others.
Last thing- I lived in Rwanda for a year and those people livin in mud houses had 16 kids and you better believe they didn't have the "recommended" diet, but they still lived and grew up just fine. So don't worry tooo much. Babies aren't like adults; they'll eat when they want what they want. Maybe your child is a snacker and he'll drink more if you give it to him little by little all day rather than at specific times? I'm often surprised at when my son decides he wants a drink. I nurse and now he's old enough to just point when he's thirsty. Wow if only he'd been able to communicate with me that easily a few months ago! When ya nurse it's easy to feed on command because you always have milk on you. Maybe you can leave a bottle of formula or water or juice around the house and he can just point when he wants it?