If you are already using pullups for nighttime, that's half the battle right there. If you are using diapers at night still, get some pullups a few days before you run out, and use the last diaper to say 'bye bye diapers!' so that he understands that it's time for him to start being dry at night.
Make sure you have a cloth-based waterproof bedpad to keep under your son as he sleeps, and have a couple of extras. Having it now and having him learn to sleep on it will help with bed changes later.
Do limit his drinks before bed, especially any soda which should not be given after supper in any case. Watch also that he doesn't drink milk after supper. Water and juices are okay but small amounts. A few sips of water before bed (after he empties his bladder) should not be harmful if he's thirsty.
Wake him up to potty before you go to bed. This will get him used to the idea that if he needs to go, he should get up rather than wet his pullup like he's done with diapers.
Once he's had a week or two of dry pullups, then you can make a happy time of switching to sleep in real underwear.
Sometimes, even after all that, you can get a child who just cannot learn to keep dry at night. Some children get to be seven, eight, nine years old before their bodies mature enough to stay dry at night. Some never do and have to have medicine to help them. This isn't the child's fault...never put the blame on a child if they are unable to stay dry at night; his/her body just isn't ready. But the vast majority of children will do just fine.
Good luck!!