I am sorry to see you have waited so long for a response. Early mornings there are fewer experts online so that may be why you did not receive a response. This information may be too late for you, but I wanted to reply so you don't feel ignored.
This drug is dopamine receptor agonist used in Parkinson's patients, it is not used in dogs so we don't have much data on it.
Vomiting is a common side effect from increased levels of dopamine.
I cannot find an exact toxic dose for dogs, as dogs weren't the target species so there isn't much research, but it appears that because most dogs vomit with ingestion they rid themselves of the drug and thus more serious effects aren't seen.
Possible side effects of ingestion include:
repeated vomiting, nausea and drooling, increased heart rate and blood pressure, and hyperactivity.
The dose he took, 2mg, is a fairly low one even for a 5 pound dog, but he may experience drooling, nausea and vomiting. If Ryder is otherwise healthy I would not expect any more than those symptoms. An elevated heart rate and blood pressure are more likely with a larger dose.
If he did not vomit the pill I would not worry much and take a wait and see approach.
If he vomits a few times once his vomiting stops to settle his stomach today you can give either:
1) Pepcid ac (famotidine) at a dose of one quarer of a 10mg tablet per 5 to 10 pounds of body weight every 12 hours
2) Prilosec (omeprazole) at 1/4 of a 10mg tablet per 5 to 10 pounds of body weight every 24 hours
These are acid reducers and should help him feel better. They can be given for a few days if necessary.
I also recommend feeding a bland diet of 1/3 boiled, shredded white chicken and 2/3 boiled white rice. You can mix in low salt chicken broth or warm water to make the food softer and easier to eat and swallow.
If he continues to vomit despite the acid reducer being given he should see a veterinarian on an emergency basis for intravenous fluids and supportive care.
He should start to eat and feel better as the nausea sensation resolves but we do want to keep a close eye on him as if he does continue to vomit he could dehydrate himself.
If you notice a racing or irregular heart rate/rhythm (very unlikely) then he should be seen by an emergency clinic for treatment.
You can feel his heart by placing your hand flat on the side of his chest just behind his elbow. Count the number of beats in 15 seconds and multiply by 4 to get beats per minute. If his heart rate gets up higher than 170-180 beats per minute consistently he should be seen by a veterinarian.
Please let me know if you have any further questions. Again my apologies on your wait.