No. It's antibiotics, not poison. The concentrated medicine will flush out overnight. Sometimes even with humans, doses are doubled intially to speed up the treatment of infection. Don't worry. Just make sure he has plenty of water to drink.
Not really. If you have some plain yogurt you can give some of that to counteract the antibiotic killing the good bacteria, but you don't have too. Your dog may get a bit of diarrhea, but some dogs do anyway from being on antibiotics.