Hi J..
I have a 6 year old boy and a 3 year old boy. The 6 year old wore pull-ups to bed until he was 5. From about age 3 to 5, he was only wet sporadically, but then when he turned 5, it was like someone flipped a magic switch and he woke up to use the bathroom or held it till morning. My 3 year old has been potty trained for a year and has not worn pull-ups at night for a couple of months now, with only one accident.
What I'm trying to stress is that every child develops in his own time. Your son's bladder may just not be ready yet to make it all night. Pull-ups might give him a little more confidence than a diaper, and they do make some that alert the child to wetness rather than absorb it all. I worry that if you make it too much of a big deal because it bothers you that it may begin to really bother him, and he might not be able to control it. Some kids just sleep that soundly, and if he's doing fine during the day, is it that big a deal??
Something I have done for both of my boys is to buy mattress protectors/plastic sheets. I put a mattress pad over them so it doesn't feel like they're sleeping on plastic. If there is an accident, it's much easier to throw the sheets and all in the washer than it is to scrub a mattress. Plastic liners you can get pretty cheap, and they work.
Good Luck,
L. B.