Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
If he just this, we need to act quickly. This dose is much to high for the average sized cat and the risk of side effects (especially sedation, slowing of the heart/breathing rates) is high. Therefore, if he just had this within the past 30 minutes, then we can induce vomiting now. To induce vomiting at home, you can administer 3% hydrogen peroxide orally at a dose of 1ml per pound. (1 teaspoons per 10 pounds of body weight). You can give it via dropper, syringe,turkey baster – we just want to get it in. After giving this orally, move the abdomen around or get your wee one walking about to get things mixing. This should usually lead to vomiting. If it is unsuccessful after 10 minutes then it can be repeated twice more. And if we still have no vomiting, then you'd need to consider seeing your local vet (or ER vet) so that apomorhpine (a very strong injectable emetic) can be administered just get this out of the stomach and avoid any adverse issues.
Afterwards or alternatively, you can also consider administering activated charcoal at this stage. This is available over the counter from the pharmacy (ask for the high strength version, not the one for gas) and works by binding any remaining material in the stomach. For activated charcoal, we tend to give 1-4 grams per pound every 8hrs. This can be mixed with food to be fed or with water to syringe feed (do note that it stains, so keep it away from white carpets/clothes). This will just limit how much is absorbed and reduce the intoxication risk here.
Overall, this is a much too high dose for a cat. Therefore, in this case we need to act quickly to reduce his risk here. Therefore, we'd want to use the above now. And if you cannot for any reason or are struggling, then we'd want him to the local ER vet. To find your local ER veterinary clinic, you can check @ http://www.vetlocator.com/or http://www.veccs.org/index.php?option=com_hospitals&nationid=1&Itemid=193
Please take care,