Since you're co-sleeping, you are in a great position to cut out the diapers immediately. I would focus on really communicating with your son about the fact that he won't be wearing a diaper, so if he needs to go potty, he will need to get out of bed and do it. Keep a potty right next to the bed to make this easy for him, and also make it part of his routine to pee in the potty right before going to sleep at night.
If you're pretty sure that he doesn't pee during the night, but only upon waking up in the morning, then you're pretty much home free. You just have to work on getting that first morning pee in the potty, rather than a diaper. Again, it's all about communication, and if he does happen to forget to get out of bed before peeing, it's something he will probably remember after that. I wouldn't get mad at such an "accident," but just calmly state that since he didn't get out of bed to go pee, all of the sheets will have to be changed, and that's a lot of work for mommy! So in the future, please remember to get out of bed to go pee.
If you think your son might be peeing at some point during the night, you will need to be a little more proactive and pay close attention to his patterns and signals. My son wakes up "to nurse" about four hours after he nurses to sleep, but he's actually waking up because he needs to pee. So I help him pee at that one time during the night (sometimes he insists on nursing first and then peeing - you may need to play with different ways of doing it), and then I know that he won't need to pee again until after he wakes up for the day. My son gets very restless in his sleep before that first wake-up, and if I notice this, I can sometimes put him on the potty while he's still asleep, and he will pee without ever waking up and without needing to nurse. But usually, I'm sleeping too deeply myself to notice the early signs that he needs to pee, so it takes having him sit up in bed or ask to nurse before I wake up.
Ideas for protecting your bed during the early diaper-free days can be found here:
http://www.mothering.com/discussions...d.php?t=621853