Contacting a friend in korean prison.

Recently a very close friend of mine dissapeared with only speculation as to where he was. recently we found out he’s in prison in south korea (he’s originally from the uk) and i was just wondering if there are any steps i can take to send him a letter? i’m unsure how to obtain his prison number or any other contact details. any help would be greatly appreciated as i just want to make sure my friend is okay.