Is this a potty training issue or not?

[AHP84]

My 2 1/2 year old son is completely potty trained and has been for a few months. He has no issues with using the toilet.
What has me stumped, however, is that for the last couple of months, his "pooping cycle" seems to have changed from daily early evening (between 5-8 pm) to the wee hours of the morning. Every single morning, I find that he's pooped in his diaper (which he only wears at night), and I know he must do it sometime between midnight and 2 am, because I go to bed around 11-12 every night, and at that point he's not soiled.
I don't know if this is something he is doing unconciously while sleeping, or if he actually is waking up and doing it then going back to sleep. All I know is his bedroom reeks in the mornings and I'm sick of waking up to the stench, as his room is across the hall from mine in a very small house.
For a week, I tried sitting him on the toilet before bedtime. I had success once, and the rest of the week was like a game to him. He'd sit there and goof around and try to play with me while I tried every incentive, coax, and bribe I could think of. At the end of the week, I decided to give up after spending the last night with him on the toilet for nearly an hour trying to get him to poop before bed.
This week, I'm trying to change his cycle by giving him fiber crackers, bran muffins, extra juice and fruits early in the day so as to hopefully have it all run through his system by early evening. Well, I had luck once so far. Two nights ago I had him in the tub and he told me urgently that he had to go. I barely got him on the toilet. And he still pooped in his diaper that night (although I must say I saw that one coming).
I tell him every night before bed that if he wakes up and has to go he needs to call me and wake me up to help him. We even "played it out" so he'd get the idea. No luck.
I've tried rewarding him when he wakes up with a non-poopy diaper. He doesn't seem to care because to him it's just another morning, whether he's pooped or not.
I just don't know if he's doing this because he knows he's wearing a diaper at night and therefore he CAN poop in it, or if it truly is beyond his control, as he is mostly, if not completely, asleep so it's more like an issue of wetting the bed and therefore something he'll outgrow in time?
Should I let him sleep in underpants (which I dread the thought of, especially when it comes to washing poopy sheets and blankets every single day if I were to do that)? Should I just ignore this stinky problem and hope it passes with time, as bedwetting eventually does with most children?

Has anyone else here ever had this problem?!

[anxiousmomtobe?]

I really don't think this is anythign he has control over.  I would ignore it and it will pass in time.

[AHP84]

I just wanted to post an update to my original post, in case it might be a help to any other parent out there with a similar problem as the one I've had with my son on this issue.
I have solved the problem, or at least figured out how to get through it.
Friday night before putting him to bed, I had him on the toilet yet again to at least make an effort. Well, it ended up a power struggle (as usual), because he had gotten to the point where he wouldn't even try to go anymore. So I asked him, "Where are you going to go poop, Trevor? Are you going to go in the potty or in your diaper?" He looked me right in the eyes and said quite confidently, "I'm going to poop in my diaper. Put my diaper on, Mommy."
So, I then decided to face the fact that I was going to have to do a lot of laundy for the next few days/weeks/whatever, because he answered my question: he was waking up and pooping in his diaper at night because he knew he could. And knowing he'd be cleaned up in the mornings.
So I put underpants on him for the night (and he had a complete meltdown about that, and screamed at me to put a diaper on him for half an hour).
Well, as expected, he wet the bed twice Friday night, but didn't poop. But he woke up and went potty as soon as it happened. Saturday night, he wet the bed and pooped, but he woke up right afterwards to go potty and clean up.
Last night, he woke up and stopped himself in mid bed-wet, and got up and went potty (around 11:00 pm). A couple hours later (around 12:30 am), he woke up and told me he had to go poop in the potty, AND HE DID!!! I was so happy and proud of him, and he seemed pretty impressed with himself too!
So it's been a messy weekend, and I've done three loads of laundry, but success is such a wonderful feeling for both of us.