It is a hard question, I have had cats with FL before. I wouldn't put him down right now, but realize it will have to be done sometime. Make your peace with it, say goodbye, treat him extra gently and try to make sure he drinks LOTS of water. When he gets sick again, and he will, that is when you will want to have him put down.
It doesn't make sense to pay money repeatedly to extend his life, but you don't have to put him down while he is doing OK either. Keeping him alive but in pain is no good either.
Keep him away from other cats, as FL is spread through shared food and water. If you have other cats, make sure they get vaccinated and tested as well.
I'm so sorry about kitty, but at least you get a chance to say goodbye. Enjoy the time you have left and make sure his last days are good ones.