My son had allergy testing yesterday and all was negative. Today he is vomiting and has stomach cramping. Vomiting and cramping

2 Separate issues. This is very likely a gastrointestinal bug that will pass in 12-24 hours. This is not due to the allergy testing unless he has a very red and inflamed rash on the back where his testing was done. If not better tomorrow, make an appointment with your pediatrician.
There . There is likely no association between the allergy testing he had the day prior to the episode of vomiting and stomach cramping. He likely is suffering from acute gastroenteritis(likely caused by a virus or even a bacteris). The range of symptoms vary and can range from abdominal pain, cramping, nausea, vomiting, and/or diarrhea. This condition is usually self limiting and goes away without any specific treatment. Make sure he stays hydrated. If the pain worsens, a fever develops, or the vomiting is frequent, he should be seen right away by his primary care doctor or in an emergency or urgent care setting.