From: In my chair
As others have said, be up-front and honest with him. Do NOT try to soften the blow - I am not talking about trying to hurt him more, but do not lead him on in any way.
It seems to me that what you have here is somebody who has just lost a big part of his life, and he's trying to find a way to replace it. So, as he gets further into his rebound, assuming that he's not an obsessive personality, he should even out. Think of it as an airplane with the autopilot suddenly cutting out during turbulence. It takes a moment for the pilot to stabilize, but the plane does stabilize.
But, as I said, and will repeat - do NOT try to avoid hurting him when you turn him down. Be blunt and to the point. Let him know that you are still his friend, but tell him how you feel, and how his actions are making you feel, and then trust him to do the right thing. If he's worth your time, he will prove worthy of your trust. If he doesn't, then you may need to take more drastic steps.