I'm not a melatonin expert but (not being an expert) has never stopped me before.
So here goes ...
I've read, I believe on the LEF site, that sometimes very small dosages of melatonin can work even better than larger amounts.
Very small dosages = .3 to .5 mg
There's another factor to consider. Since it appears that your husband has trouble falling asleep AND staying asleep ... it might be worthwhile to get a time-release melatonin. They make some supplements that release part of the melatonin within an hour and the remainder over the course of 6-8 hours.
Now, if the melatonin doesn't work ... consider trying to stimulate his body into producting more of his own melatonin. Certain supplements may increase serotonin levels. Serotonin can be used, by the body, to produce melatonin. Both 5-HTP and L-Tryptophan have been used with some success for this application.
Certain foods may help as well. There's a book called, Potatoes Not Prozac
, that recommends eating a potato before bedtime. This is thought to improve sleep quality and possibly help over-all mood during one's waking hours.
The theory behind this is that by taking a potato (apart from any protein) ... will cause a rise in insulin levels and help to preferencially shuttle tryptophan into the brain - where it will be used to manufacture serotonin (and melatonin perhaps).
more info. https://www.radiantrecovery.com/4r.htm
One last suggestion ... a good head/scalp massage helps me to fall asleep.
I suspect any form of massage, that he likes, would do the trick. This not only releases feel-good chemicals in our brains but it can help us to just let go ... to disassociate from our conscious mind.
PS - Melatonin has shown activity against a variety of cancers ... including PC. So, any way that you can promote higher-levels of melatonin may be healthful.