Finding internal peace

Dear Nicholeen, this question is on behalf of my husband.
He asks if you have any suggestions on how he can increase his internal peace. He says his brain seems to run 24/7, and that he doesn't know what real internal calmness feel like. He says that he feels his brain is like an engine that never turns off. He sleeps just a little, falls sleeps late and wakes up several times during the night.
He attributes it to ADHD, for which he takes medicine. He says that medicine helps him focus at work, and become more aware of when he gets distracted or talks for too long.
We're Christian and he prays about it, but doesn't know what else he could do.
We would love your advice. Thank you!