I have taken my son to the crystal shop to pick out his own crystals. I think he may choose what he needs just as we would
He uses an amethyst crystal to put beside his fairy figurine who "protects him from monsters and shadows" while he sleeps. He has a ritual of saying this, kissing his fairy figurine, placing her on the bedside table, kissing his amethyst crystal and placing that beside her, and kissing his woolen felt fairy before taking her to bed and putting her above his head (it'd be on or under his pillow if he used one).
Since doing this little ritual, (which is so cute! ) which had evolved a little over time, he isn't afraid of the monsters and shadows.
BTW, I chose the amethyst for this purpose I think.
If your son suffers nightmares, maybe a similar ritual to alleviate the fear of sleep, and to lessen the scariness of the dreams would help?
But at the same time you would want him to, at some point when he's ready, to face his fears and process what his subconscious is showing him, so he can get through this and out the other side, stronger, more confident etc. Maybe expecting a lot there but I hope you get the idea
and maybe start small to build up his confidence of his ability to face fears, after lessening the fear with a ritual of intent.
Just a suggestion.
When I took him to the shop the first time, I picked out crystals for him to choose from (he couldn't see above the bench where they were at) so i picked a handful and asked which he liked best. I think at that time I chose crystals that were related to his month of birth/ star sign and other gentle crystals. But last time I went in I just randomly showed him crystals. So which ever way really, but I think he will be drawn to what he needs. Maybe ask what colour he's interested in to narrow it down, but let him see the colours of the crystals themselves first. (His favourite colour might be green, but in time he may find his favourite CRYSTAL colour is blue). He is a child. I believe his intuition will guide him well
I hope this helps!