Just a few thoughts.............1. You said, in the morning "when we go to pick him up out of bed." Maybe you are looming over him and he finds that intimidating. How about getting down on his level and calling to him from a few feet away from his bed? 2. Concurrently, maybe limit his water intake two hours before bedtime. 3. Have you tried a belly band or male dog diapers after you take him out but before he goes to bed for the night?
4. Do you know anything about his first six years? Perhaps he had to show submission to larger dogs, or even humans and this might be his method of acknowledging his (lowly) place in the pack order? 5.Could he have a small bladder condition and not be able to hold it for more than X number of hours? Maybe he was punished for this before you adopted him and he is rolling over to let you know it isn't on purpose and "please don't hurt me?" Rescue dogs often come with some extra baggage and it takes time to make them lose old habits, trust their new humans (you) and make them feel loved and secure. Since it is the first thing in the morning, it may take him a few moments to remember he doesn't need to be scared anymore.
Good luck and thanks for adopting a rescue!!!