First let me say sorry, when I answered your question the "already tried" had not posted, so I didn't get that information to reply to that.
If I'm understanding your question;
Taking the hydrocodone won't effect being taken with Benadryl, other than the possible drowsiness effect to be increased.
I don't know your sons weight, but I will assume under 100, even if he took a double dose of benadryl, it wouldn't give an overdose effect. If however taken with the hydrocodone too, since hydrocodone may also have ibuprofen in it too, then it's possible he may have a reaction to that then. Which then should at least be 5 hours apart. (between the two meds)
Is the arm wrapped with a temporary splint or permanent cast? Blowing a hair dryer on cool setting can help. If the itching is severe, then his doctor should be notified to make sure there is not swelling that is causing any lack of circulation.