"telling" should probably be "tell." "Why not tell him" sounds like more correct English than "why not telling him."
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".
I had no idea how complicated the rules were for putting "to" before an infinitive!
It's a literal translation. "Why don't you tell him?" would literally be ?למה אתה לא אומר לו
Is it possible that this would be better translated into the passive voice "why is he not being told?"