Do you know the brand of the chlorine tablet?How long ago did he eat it?How much does your dog weigh?
Was it the kind of tablet for pools?
Thanks for answering my questions.
At this point, it's unlikely that if we induced vomiting we'd get anything up, plus Chlorine is just bleach, so inducing vomiting could actually make matters worse.
If this were my dog, I would have him seen by a vet immediately. The problem with chlorine is that as it filters out of the body through the kidneys, it can cause serious damage, not to mention the possibility of ulcerating the stomach and other issues. Your dog needs to have medication to coat and protect the stomach and IV fluids to flush the chlorine out of his system safely.
Because of this, I have to strongly recommend having him seen by a vet. I wish I were able to give you an easy over-the-counter answer, but an in person check is best.