You'll probably receive a lot of suggestions for melatonin - I've had doctors recommend it and warn me to stay away from it.
Personally, I think there might be a few things going on that warrant a conversation with a physician:
1. sleep apnea
2. Restless Leg Syndrome
3. Anxiety to prevent him from falling back asleep
Since I don't know your husband and am not clinically trained in sleep issues, those are just suggestions on possible causes.
Please remember that almost all herbal remedies are NOT FDA approved or monitored, so their safety is unknown.
Here are some resources that may help you from MayoClinic:
http://www.mayoclinic.com/health/melatonin-side-effects/A...
http://www.mayoclinic.com/health/sleep-aids/SL00016