Risperdal (the brand name for risperidone) is not a veterinary drug, however studies have been done on Beagles for the drug.
Overdose symptoms include lethargy, weakness, muscle tremors, ataxia and seizures. It is too late at this point to induce vomiting, so if you absolutely will not take him to a vet, you need to watch him closely for any of the symptoms I listed above. If you see them, you need to not eat for a week if necessary, but you MUST take him to a vet or you will lose him.