Thank you for the information.
I am glad to hear he is doing well on the Atopica I have seen such amazing results with this drug
The purina dog food is very high in fat and his system is likely reacting to it. A sudden diet change or foods that are rich or high in fat can cause irritation of the digestive tract as well as inflammation of the pancreas.The obsessive licking can be a sign of nausea.
If there is some mild inflammation in the pancreas this can be very painful.
The first line of treatment for vomiting and diarrhea is to with hold food and water for 12 hours. You can offer ice cubes as a means for him to stay hydrated. If you have a way to get liquid pepto this may help coat the stomach so he feels better.He should have nothing by mouth other then the pepto if you can get the liquid.
After the fast you can offer a bland diet of boiled chicken and rice fed in small meals 2-3 tablespoons several times a day.
If he is able to hold down food and water tomorrow you can try giving imodium for the diarrhea as some owners find this helpful.Here you can find information on this drug and the pepto including the dose.
Vomiting and diarrhea can be signs of problems with the kidney or liver in an older dog so if he continues to vomit you should have him seen by your vet.
I hope this has been helpful, please reply back if you have additional questions or concerns.