I dont understand, do you want something on his collar so you can hear him? Or do you want something so he can hear himself? Or do you want something so that you can call him to you?
If you want him to come to you, but dont want to yell, try a cowbell... Ring the cowbell while doing what you do when you want him to come to you, like patting your leg.. after he comes, or if he doesnt come and just looks at you, set his food down on the ground. Keep doing this at feeding time, and he will come to associate the loud bell with food, or that you want him to come to you. ONce he starts coming when you ring the cow bell, ring the bell somewhere else in the house, with a cat treat or two on hand to give him when he comes. Pretty soon you will just be able to ring the bell and have him come running. If you dont know where to get your hands on a cowbell (to be honest I am not sure either, but if you know what a fleet farm is or have one, try that, or go to your local k-mart, wal-mart, or whatever you might have and try to find a really loud bell. Sometimes they might have loud things in the children's isle. Maybe a whistle or a cheap musical instrument like a recorder would work too.