This was on another website re this sort of thing:
Calm down spray
If you get Bach Flower Remedies 'Rescue remedy' and 'Star of Bethlehem' put two drops of each in a 30 mL bottle, top up with filtered/bottled water, (never tap water) put into spray bottle and spray it around him. Spray him 3/4 x per day. Just a light misting is all you need, don't soak him. It will help calm him and the Star of Bethlehem will work on the trauma
They were talking about help with thunderstorms when they mentioned trauma!