Now the problem with any grape product (including the juice) is that we do not know what the toxin in grapes actually is. This means we don't even have safe exposure ranges for dogs and toxicity appears to vary by grape species and batches. Therefore, we do need to be aware that while this sounds low risk (with him being larger and this being a small amount), we just cannot be 100% sure (no one can).
Therefore, since grape toxicity can cause damage to the kidneys, we are best off erring on the side of caution. As this is a liquid ingestion, I would suggest that instead of inducing vomiting, we could 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 8 hrs. 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. As well, just to note, you could alternatively have him to his vet for IV fluids to flush any toxin out.
Otherwise, we are going to need to keep an eye out for any increased thirst or urination (especially of dilute urine). Seeing these would be warning signs that the kidneys are struggling. To just monitor this, you can consider collecting a urine sample for your vet to check the concentration of. As long as it remains normal over the next week, we are likely in the clear.
Overall, this is a low dose and we'd expect it to be low risk (especially as grape juice isn't usually just grape). Still with the nature of this toxicity, we do need to tread with care. Therefore, I would advise the activated charcoal here +/- IV fluids with his vet. If we do one or both, we can just ensure that we are reducing any risk for Riley.
Please take care,