Now the Pelargonium plant species are considered to be toxic to cats. Though not severely toxic as we see with some other plants (like Lilies). Specifically, this one is nown to cause GI upset (drooling, appetite loss, vomiting, diarrhea), depression, lethargy, and we can see oral or skin irritation where it has been in contact.
Now if he has only had some diarrhea after eating this 8 hours ago, I agree that the volume he ate would have been small. Therefore, at this stage, we'd not be overly worried and would just want to use a bit of supportive care while this runs its course. Therefore, at this stage, you can consider offering a light diet option for a few days. Examples of an easily digestible diet include boiled chicken, boiled white fish, scrambled egg, or meat baby food (as long as its free from garlic or onion powder). Ideally, we want to offer this as small frequent meals to keep his stomach settled. And as these are easy to digest, they will reduce any nausea and diarrhea for him.
Further to this, if you think he has a bit naseous, then you can also cover him with an antacid to keep him stomach as settled as possible. There are a number of antacids that are available over the counter and pet friendly. I would advise only treating with one, but the two I tend to use are Pepcid (More Info/Dose) or Zantac (More Info/Dose). These are usually given 20 minutes before offering food (to allow absorption) and of course you want to double check with your vet if your wee one has a pre-existing condition or is on any medications you haven't mentioned.
Overall, this plant isn't cat friendly but the signs it causes are not severe. And if we are only see mild diarrhea 8 hours on, this is likely all we will see. Therefore, we'd just want to offer a light diet, monitor, and move the plant out of reach at this point and it should just run its course.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best, Dr. B.