You have to let him grieve...lots of attention and treats may help. I had the same thing happen back in August..my female tiel died and her mate just grieved and grieved. He looked for her and called for her..He settled down after about two weeks..but still continued to look for her. I am sure if she came strolling through the door right now, he would be the happiest tiel on earth..unfortunately, this is just something you have to wait out.