I have a best guy friend and I told him that I loved him too.
But, I tell ALL my friends I love them. So, how i went about it was to make sure that he would hear me tell this to my friends a few times before I told him.
That way, if it DID make him uncomfortable ( which you can immediately tell) then you can always just say " hey. you are my friend. And just like alllll my others, I love you just the same"
If it gets sticky after that, you have several examples he has experienced of you telling your friends how much you just love them and love them in your life.
This may not work for everyone though . But for me it did. And that is because i am the type of person who -will- tell my friends that I do love them because it is true. Im not shy about saying that and if you are my friend you will know that I love you. So hearing words like that often from me let him know that he did not need to be 'afraid' and that he KNEW even as a friend I loved him .......... which is the safest way to say something like that.
And the undertone of the words were accepted as well.
Sometimes, that is the best start.