how to tell my bro about masturbating

My advice would be to just be honest with him.  Most people figure it out by themselves eventually - but talk about as little or as much as you're comfortable with.  That would depend a lot on your relationship with him.  But - since he came to you about it...that says a lot about how much he trusts you.  I think the most important thing is that you let him know it is normal, perfectly ok, and that he shouldn't be ashamed at all for it.