My cat is peeing in his sleep. What are the possible medical causes?

It is possible that he has a urinary tract infection, urinary bladder stones, or a tumor in his bladder that is causing this, though often we see urgency to urinate, pain with urination, etc. Its also possible that its a neurologic problem resulting in him not knowing that he's urinating in his sleep. Does he have any other symptoms?
Ideally, take him to your vet for an exam. Your vet will probably recommend doing a urinalysis and perhaps some bloodwork. If a cause is found, the treatment is targeted for the cause. If no cause is found, your vet may recommend a prescription that helps him to tighten his urethra (Proin). However, this is a very rare problem to see in male cats; so we would not want to assume that something like that would help if there is an underlying cause. Additionally, to use a mediation like Proin inappropriately could cause a problem with preventing him from urinating when he should.
I hope thats helpful, sorry I don't have more specific answers for you!! Best of luck.