I think that you're very lucky to be able to have such a wonderful friend in your life.
However this is a decision you'll have to make. If your best friend loves you back, I'm sure he is already aware that best friends last forever but lovers don't always.
If you think you love him enough to tell him your feelings, you need to be ready to sacrifice everything you have ever had with him. I'm not saying that things won't work out. If it does, then it's wonderful but if it doesn't, those are the consequences you'll have to face.
No, my post sounds almost negative. >.> In a positive way though, I think you should just monitor the situation for awhile before you decide.