Hi S. - a consult with your doc could definitely help to see if the bed-wetting and the constipation is something they can help with.
As far as the constipation, make sure he gets lots of fiber along with lots of fluids. It's amazing how quickly a guzzle of prune juice mixed with apple juice takes effect too. Talk to your doc, but we were able to give our son Milk of Magnesia when he had a bad bout of constipation after surgery.
We also make our son take time to have a bm as part of his bedtime routine, otherwise he will wait as long as possible too. I really cant figure out a reason why. At night, he takes in a book or some markers does his thing. If nothing happens, it's no big deal, it usually happens the next day. My son was withholding for a couple of days and was starting to get messy trying to hold it in. He sat on the potty for a long time but just didnt want to go. I knew he could be done and over in about 30 sec. so I said if you can make poop in the next 3 minutes, I'll give you a dollar. Sure enough, 30 seconds later, he was asking for his dollar. I dont recommend "poop pay" but it was a good lesson for him that he could do it with just a little motivation.
My other thought is that the bed weting might have one correlation to the constipation. When the colon is full it can't help but put pressure on the bladder. When boys have a bm, it's nearly impossible to keep from releasing urine - it's just how the anatomy is arranged. If he is a deep sleeper and he has pressure from his colon, then he might be more likely to wet the bed.
I hope that helps to give you some ideas -