My son developed hives and I wonder if it was because he ate some green bananas. Can hives be caused by a banana allergy?

Generally not. Hives due to food allergy will typically start between 10 and 60 minutes after eating a food, and last for several hours - typically gone by the next day. They will not come back after that, unless the same food is eaten again. Recurrent hives regardless of any recurring ingestion or exposure should be investigated by an allergist.
Banana allergy hives. Yes hives can be caused by any food allegy and if your child is allergic to banana then it can cause hives as well.To be sure you have him tested by your pediatrician or an allergist.