Have you tried oral Benadryl? I have one son who deals with it a lot. And the oral stuff does help sometimes. The other things you're trying don't do a lot for him either--takes the edge off a bit, but not complete relief by any means!
Also, for future reference, keep some Tecnu wash on hand (we got it at CVS). It's a liquid soap that you can use after you're exposed, before breaking out. When you think you might have been exposed, wash the area with this stuff and it helps remove the oils from your skin. They also make a gel to put on the rash, but we found it wasn't a lot different than Calamine.
He has had to do steroids a couple times when it's gotten on his face, near his eyes, or when he's had a really, really bad case. The doctor will give him a short course--like five days--and it's enough to really start the healing process. I'm not a huge fan of steroids, but sometimes you just have to!