How do I stop my dog howling in his sleep?

The only thing I can think of is just to wake him up nicely, like a soft pat on the back or something till he wakes up, or calling his name till he wakes up and then comforting him and telling him it's ok. Dog's can't really speak obviously so he won't be able to tell you what it was about but maybe just some comforting will help. And other than that just try to get some sleep in between these howling nightmares and perhaps if he's comforted after them on a regular basis they might happen less often or hopefully not happen at all anymore.