I would not use the bitter apple for a baby. Yes, it would work, and it is safe for humans, but it is pretty harsh for a baby (I have, however, considered putting a little on DS's fingers to keep him from biting them-yes, I said biting, he doesn't suck his thumb/finger, he bites them). I have been sprayed with it on acccident, and it is pretty powerful stuff (now, if only it would keep my cat from chewing our electrical cords-he somehow actually likes the bitter taste!). There is some kind of a pepper component to it that could cause a bad reaction in a very little person, and it will burn eyes.
Maybe one of those table/fireplace bumper wraps? It may not keep him from chewing, but it will help protect the furniture (of course, if he likes the taste of the wood, and that goes away...). We had corner bumbers on one of our tables. It not only served to protect DS from injuring himself on the sharp corners, but it also protected the table from him chewing on it. While the bumbers had little teeth marks in them, we never saw any pieces missing, they are pretty well constructed (and he did a lot of chewing!).
How old is he, is he teething? Maybe try some other type of teething toy?
Also, maybe talk to his doctor to see if there could possibly be some kind of deficiency in his diet. I know it sounds strange, but it is possible. People get cravings for strange things if there is some kind of nutrient missing from their diet.
Good luck!