Oddly enough, Force Fetch doesn't have much to do with actual "fetching". It is a method to teach the dog how to respond to pressure. Now, on to answer your questions. The absolute best information on FF is Smartfetch by Evan Graham (
http://www.rushcreekpress.com/page1homepage.html). The poster above is somewhat correct in a solution to the problem. What has happened is that the dog has figured out the routine (whether you were trying to establish one or not). I typically don't teach "hold" as I consider the delivery to hand a part of the retrieve (the retrieve is not done until I take the bumper out of the dogs mouth).
Here are some steps to fix the issue:
1. Remember, the key to fixing this (and many other issues) is escalating pressure
2. Put the dog at sit and put the bumper in its mouth, as soon as he spits it out (and he will), say "No" firmly and put the bumper back in his mouth. IMMEDIATELY PRAISE HIM (SCRATCH EARS, RUB SIDE, WHATEVER WORKS FOR YOU).
3. When he spits it out again, repeat the above steps, but this time the "No" becomes more firm, and the "placement" of the bumper back in the dogs mouth is more dramatic. Once its back in the dogs mouth for a couple of seconds, repeat the praise.
4. The idea is to build up the duration of the dog holding the bumper by praising him while he's holding it.
5. Don't "wear this out". Do it 2 or 3 times in a row. When he holds the bumper for a couple of seconds, after you praise him, take the bumper out and throw it for him to retrieve. Don't worry about him holding at sit at this point. You just want to "break the pressure".
6. While he's bringing the bumper back, praise him up good and back away from him several steps until he gets to you. If he drops the bumper, put it back in his mouth and praise him up. If he drops it, turn your back.
Hope this helps. If you have any other questions, feel free to email or PM me.