Great. Thanks for the additional information. I really appreciate it.
As a general rule, the average dose size of this kind of medication in dogs is 0.1mg/10 lb of body weight, which means that your little dog would get 0.1 - 0.15 mg twice a day. In studies, it's shown that an over dose of up to 3-6 times more shows very little problems, while over doses over that can be hepatotoxic...meaning they impact the kidneys.
The problem with kidney damage is that a dog can be sick for several days before they show any symptoms, and once they do, it can be too late to fix things.
Because of this, the absolute best thing to do is to give your vet a quick call and ask if they want to run any blood work and check those values, or if they want you to just monitor him at this point.
Hopefully he'll end up being just fine, but since he got such a big dose and he's such a little dog, it's going to better to err on the side of caution and touch base with your vet today, rather than waiting until he gets sicker this weekend and you end up having to go to an expensive emergency vet.
I hope this helps.