My 1st thought was benadryl cream, but you have already tried that. I don't know how long you mean by, "All helped, but only for a very short while" Remember, anything you use will have to be reapplied every few hours for the next few days, until it subsides. Nothing is going to make it disappear in just one application. A benadryl pill at bed time should help him sleep and not scratch at night. May also use during the day, but may make him drowsy.
Next thing I'd do is try putting a few drops of water on an aspirin and cover the spot with the aspirin paste. You could also try some topical anesthetic, such as aloe with lidocaine, or a sunburn or hemorrhoid relief spray that has benzocaine in it.
Probably too late for this time, but in the future, if you tape a penny over the sting for a few hours, the copper will draw out and neutralize the poison.