I think you can not push him...a year is not that long to grieve...if they were engaged than the relationship was at a peak and for her to suddenly die is a big blow.

He clearly said he is not ready and does not want to hurt you.

I hate to say this but I don't think you are being "used" because I would hate to think that you would allow yourself to be used..and I think as a man....if there are feelings there for you and sex is on the table he is not going to turn it down..after all you are a close friend to him and I'm sure he would rather have sex with a close friend when he is down and out than go find some random stranger.

I really think looking from the outside that I would step back from this situation. I would find time to spend with other friends and family members and do not make him the center of your world. You can very easily push someone away entirely when they say they need to not be in a loving relationship right now and then you keep going down that road by sleeping with him...and having that platonic relationship with him.

I see you getting hurt in this situation but I don't think that blame can be put on him as he has been honest with you about his feelings.

Like I said..step back find other activities besides this gentlemen....maybe if it is such a good connection in time you two may end up dating but now is clearly not the time (in his words) to pursue the avenue of becoming a "couple".