I've never been in your position but an idea that I had and would use if one of mine went missing is to park my car and leave it in the most prominent space outside a large supermarket / retail park - you can plaster every square inch of glass inside with 'lost cat' posters so that it looks really odd and people can't fail to have a look. You can potentially 'advertise' the fact he's missing to thousands of people in just one day and also the shoppers may be from various different areas so helps spread the word further afield.