How to keep 14 year old step son from scratching himself

My 14 year old step son would scratch himself to self harm. He had a rough life. His mom and him both went missing when he was a 9. Turns out her boyfriend was pimping them both against their will. His mom ended up getting killed by her boyfriend. He's had 3 therapist and he does improve, but sometimes he takes 10 steps back after taking one step forward. He tries not to scratch himself now but in his sleep if he's have a night terror he will scratch up his entire arm. My hubby woke him up from having a night terror because he was screaming and scratching up his arm. My husband is wanted to make him where socks on his hands while he sleep a but that's stupid and silly.