Boy, that's bad. Hopefully you can find something that'll get him some relief without messing him up some other way.
I have hopes for the Tri-Hist granules. I just top-dress a tablespoon full of it over her feed once a day. It's supposed to last 12 hours and you can give it twice a day, but I can only get out there in the evenings. She doesn't like it much, but she will eat it, just over plain oats, which is all I'm feeding these days. I got it from the vet and it was about $40 for about a quart-sized plastic jar full.
I tried a product called Hist-All, it has one of the same indredients as the Tri-Hist but doesn't have the pseudoephedrine. Let me know if it works for you. The Hist-All worked on Tuffy's cough but didn't put a dent in the itching.