L.G.
Don't worry about transitioning from the sippy yet--it helps prevent spills! If anyone gives you grief, point to how many adults use sippy cups: from Starbucks, from "sport cap" water bottles, etc: we ALL use sippies! (hey, put his milk or juice in a cup that says "Starbucks" and you won't have to worry about anyone giving you grief because of the sippy; they'll have something else to give you grief about LOL!)
When you really do want him to transition from sippy cups, here's one I read (but didn't need to use): cut away a tiny bit of the top of the sippy every day or so, making the top a bit more open every time; eventually it's almost completely open, he's probably developed the fine-motor coordination to not spill his cup anymore, and it'll become a moot point. Best of luck!