Switching from the bottle to the sippy cup is a big deal for our little ones...my doctor told me it is more work at first for them to use a sippy cup because it is a different motion/tongue position then they are used to, but if can get rid of the binky then this shouldn't be so hard.
With all 3 of my children I started giving them juice in their cups at about 6 months - since juice is sweet I figured it would be more of an incentive and when they turned 1 the bottles were gone and then they got everything in a sippy cup. it was a bit of a transition, but no one starved. :-) I think it helped that I started the sippy cup earlier.
I am not sure how long you have been giving your son a sippy cup - if you started earlier, like I did, then I would suggest trying it cold turkey. If he hasn't had that much experience with the cup, then I would suggest trying putting his juice in it and gradually putting his milk in it as well and after a few months or sooner if you feel he is ready, then going cold turkey.
Good luck ~