Most Helpful Girl
I have answered This Same Question many a time here, dear, so to make it light and sweet for now just Explain to him that you have decided to keep your relationship that of Friendship and you care about him but just feel that friends is the way to go for you and this is how you Are... Feeling about him.
It's not necessary to have to Explain at this moment about the 'Someone else." When it ends, down the love line, he will see this, of course be hurt but that is something that no matter what you tell him or what 'Other methods,' he would be 'Hurt' anyways Any day.
Good luck. xx