Hypnosis for Language Recovery

Someone I know, upon finding out I am a hypnotist, asked me if I could help him recall the language he spoke as a child. Around the time he was six years old, he changed countries and learned to speak only English--and now he has consciously forgotten  all of his first language. It's easy enough, of course, to regress someone to the age where they spoke only their first language... but what information sources are there on working with someone in hypnosis so they will recover full conscious use of the forgotten language?