If he's open to the basic practices of magic you can try to teach him some basic meditation and other calming exercises which do work quite well to help someone overcome fear regularly. If not then you can certainly try a spell, I don't know one myself but you can always create your own. Aspects that might help you are;
Candles - blue can help promote tranquility
- gold can help with confidence and courage
- orange can help with courage
- red can also help with courage
- purple can help with self-esteem and inner strength
- white can be used for any purpose
- green can help with security (although that would be more about separating him from his fears rather than conquering them)
Crystals that promote confidence and courage are agate, amethyst, aquamarine, blood stone, calcite (specifically helps to calm fears), carnelian (again specifically prevents fear and promotes calmness), diamond, jade, lapiz lazuli, ruby,tiger's eye, tourmaline and turquoise.
Herbs that help promote confidence and courage are black pepper, cedar, clove, columbine, fennel, ginger, mullein, musk, onion, sweat pea, thyme, tonka bean and yarrow.
(I took the above from "The Pocket Spell Creator" by Kerri Connor which I happened to have handy)
I would mention though that talking is also a very effective cure for such things and as the fears are caused by quite a long time in prison then a spell or a "coat of whitewash" so to speak may not cover it completely. He may need to do a fair bit of soul searching so if he doesn't buy into meditation then try talking to him and if he's open to it perhaps suggest some version of counseling.