Without going into more detail about the cause here are some general
Stress relievers, that might help, depending on the source of his insomnia:
Warm/hot bath with calming essential oils or epsom salts to detox
Light exercise early in the day
Take a Walk around the neighborhood/trail, near a body of water (neg. ions)
Talk to you about the reason for the stress and get a plan of action to help solve the cause(s)
Beer and sex works for some guys
Get a therapeutic spa type massage
Have him take a personal day off if he can from work, and do something he enjoys. If he's not working and that is causing the stress, maybe he needs help looking up 'help wanted' ads, or putting out resumes, etc?
If he's having nightmares about the stress causing issue, maybe he needs to talk with someone other than you (a buddy, sibling, professional, etc) to get some fresh prospective on it??
Guys are tough, they usually bottle up their emotions and it's hard for them to take the down time they might need to resolve the problem or de-stress enough to be able to sleep good again.
and good luck!