I don't know if you made contact yet or not but I do have personal experience of this and found I had a half sister I knew nothing about. I wasn't aware of another sibling but through some unexplained papers and an off the cuff remark from my gran, who wouldn't then say any more, I decided it was worth investigating. All I had to go on was an address of a farm and the name of my mother's first husband. I wrote a short letter explaining that I was researching into my family history and was writing to everyone of that surname in that area (only 12 people). Eventually, last of all my half sister replied and the contact was made.
I agree with everything that has been said by other posters, the SAE is a great idea and not being specific about him being your brother is also good, it may make him more curious rather than shocked, if he doesn't know.
Also, everyone has given good comments on the outcomes which could happen. The only warning I would give is from my own point of view. Our first meeting went very well, we met half way between our homes at a pub for lunch (with our husbands). After more meetings it became clear that we had nothing in common and she craved more attention from and of my family, than we were prepared to give. We don't see each other any more and only exchange cards, the good thing to come out of it was that she got to meet our mother before she died.
I hope for your sake that it works, I was so excited about having a sister, but be prepared that it may not, although I would say keep that thought well in the background, because it's more likely to work out than not. Good Luck
