To him, he weigh 20lbs, will this cause damage to him?

If your dog is generally healthy and not too elderly or pregnant, one dose of that amount is probably OK, but you shouldn't give any in the future without OK from your vet. Why did you give it?

I agree with Joy, if this is a healthy dog this one incidence should be fine. In fact, at your dogs weight an overdose amount would have been about 675 mg (roughly).
Just keep your eye out for things like, vomiting, jaundice, gray or pale gums and low body temperature.
And it is never a bad idea to just touch base with your vet and let them know your concern.