A school friend saw blood on my sleeve one day and went to the head of year. She then phoned my mum, who, bless her, had no idea and nearly crashed the car (this was a long time ago, when mobiles were only really quite new).
I'm sorry I didn't pre-empt this but I really didn't want anyone to know.
Nowadays, there's only really scarring to worry about, barring the odd scratch. If 'm close enough to someone, they'll eventually notice the scars - or will figure out there's some reason I hate my arms and keep em hidden. If I'm close enough to someone, I'll explain.
With boyfriends etc I'll hide it and hide it for as long as possible. If they're worth sticking with, I'll be upfront in a 'there's something you should know...' way.
I think write him a letter and give it face to face if possible... if he's not the one causing the problems, make it clear from the outset that you're not blaming him and that you just want support - and I'm sure he'll take it as well as can be expected.