Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry that sweet Koji is not feeling well...poor guy!! He has certainly had a very hard start to life and I am very impressed that you are taking such great care of him. It is unlikely that Koji has chocolate toxicity since there is not much chocolate in ice cream but the large amount of sugar is probably affecting his system that is so fragile. For now, the best thing to do is to follow these instructions:
Fast Koji for 12 hrs to allow the digestive tract to calm down and heal.
Give him Pepcid AC or Famotidine at 0.5 mg per pound every 8 hrs. That can be purchased over the counter at the pharmacy.
Give only small amounts of water during the fast or ice cubes
After the fast, feed rice and boiled chicken (2/1)
If the symptoms have resolved, then wean Koji back onto regular food. If the they persist especially during the fast, take your sweet guy to your vet for an exam and lab work to determine the cause for this. This is extremely important since this can cause dehydration or other more serious concerns.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, ***** *****