Hello, I'm Dr. Deb.
I'm sorry that I was off my computer when you requested me but I'll do my best to help you and Duncan today.
It's certainly possible that the apples are at fault for his gastrointestinal issues although there are other possibilities such as something else he might have eaten, viral/bacterial infections, foreign body, inflammatory bowel disease...just to name a few examples.
Regardless, if he's otherwise acting his normal self, then the following is what I'd do if this were my case.
1. I'd give over the counter pepcid ac (Famotidine) at a dose of 1/4th mg per pound of body weight twice a day on the off chance that he might still be nauseous.
2. I'd withhold food and give everything a chance to settle down for at least 8-12 hours after his last vomiting episode.
3. Then, I'd offer him a bland diet of boiled chicken and 20-minute white rice but only a small amount at one time.
4. If he is not currently taking any aspirin or nsaid products and the vomiting has subsided, then you can give him Regular Strength Pepto Bismol. The dose would be 1 ml per 10 lbs of body weight 2-3 times a day.
For example, if he weighs 50 lbs, then you would give him 5 mls or one teaspoon.
Pepto bismol should not be given to vomiting dogs since it contains salicylates (the active ingredient in Aspirin) which can irritate the stomach and potentially worsen the situation.
5. I'd start him on a good quality, canine probiotic such as FortiFlora or Resources Protegrity GI. These products can be extremely useful for gastrointestinal issues (and to help strengthen the immune system as well). These products may be available at local pet/grain stores or can be purchased online.
If he continues to vomit and/or becomes lethargic and/or doesn't want to eat and/or his stools continue to be loose for more than 2-3 days, then a vet visit may be prudent.
I hope this helps although, again, my apologies for the delayed reply. Deb