What kind of sippy cups are you using? We use the Nuby cups with the soft silicone spout, and they're not totally spill proof like the Playtex cups and others with valves, but I think Nuby is easier to drink from, especially if sucking is a problem. You could also try a regular cup or a straw cup.
To get your son to drink milk, you could try mixing different flavors into the milk like strawberry, chocolate, Ovaltine, or even yogurt smoothies. Once he gets used to drinking that, you can gradually decrease the amount of flavor so he's just drinking pure milk. You could also try soy milk, rice milk, or goat's milk. Only problem with soy and rice milks is that they have less fat than cow's milk so you'll have to give him other sources of good fat in his diet like olive oil or avocadoes.
My daughter went through the same thing at age 1, where she refused to drink cow's milk so we eventually got her to drink vanilla Silk soy milk out of a cup, then started mixing that with whole milk. We gradually decreased the amount of soy milk so now she drinks the recommended amount of milk everyday. You just have to keep trying because it takes little ones many tries until they get used to something and decide they like it.
Like the previous mom said, yogurt is great, not just for the calcium but for the good bacteria and it also contains water. To help keep him hydrated, you could also mix extra milk or water into his food.