well there are many ways...
usually he'll figure it out on his own (ouch!)
however, if u insist on talking to him, lay it gently. VERY gently. for instance, tell him that you know that he loves you, that you appreciate it etc. however no matter ho much this means to you, you don't feel the same way.
stuff like that. it'll break his heart anyway, but let him know that it's not your purpose, you just feel like being honest because you want him to move on & fine the right person for him.