If you wanted to use the progressive "telling," it would have to be "Why aren't you telling him?" I also think "Why don't you tell him?" is a good translation; although the Hebrew uses the infinitive לומר, the subject "you" is implied.
The "to" isn't always used in English. I'm not sure of the exact rule but I know you can say "I want to be happy" and not "I want be happy". But you can say "you should be happy" and not "you should to be happy".