I know that whatever you do, do it SOON. Over the next couple months is when he'll get REALLY attached to it.
I have a friend who did the switch "cold turkey" with each of her 5 girls. They can't hold out for long, and as long as he's eating, he won't dehydrate. Maybe be sure to offer "watery" foods during the transition -- watermelon, applesauce, etc.
We actually began the transition with ours at 11 months but offering cups during the day and a bottle just in the morning. We just dropped the bottle around the first birthday and it wasn't an issue since they were already used to cups. Our favorite cups for the transition were the old fashioned Tupperware kind that have just a lid with a spout (no valve). There are other brands -- the Take 'N' Toss are good, and Especially for Baby make some.
http://www.toysrus.com/product/index.jsp?productId=240364...
The cups with the no-spill valves are made so that you have to suck pretty hard to get anything out (I personally tried them all because ours also refused them at first), so that's why we went with the simple ones.
Good luck!