I’m in the same situation. I have a sign that says “we know you would be here today if heaven weren’t so far away” propped up on a chair for the ceremony. I also bought an extra tie that matches the groomsmen ties and that will be draped over the chair. Of course, his chair will be right next to his parents in the front row. I ran this by my fiancés mother and his aunt who he’s fairly close with and she said the family would appreciate it. She actually made the sign for me. It’s going to be somewhat of a surprise. My fiancé is sentimental too, and I love that about him. Even though his brother would have likely been Best Man I still like the idea of reserving a chair. Best wishes!