Put on gloves and long sleeves. Wash him thoroughly with soap to remove any residual poison ivy oil. Be careful - he can transfer the oils to you! Since he's licking so much, I'd not try anything topical at first, since he's just going to lick it off. You can give him some Benedryl - plain diphenhydramine, with nothing added. The dose is 1-2mg/lb, so if he's 50#, the dose would be 50-100mg. I believe commercially available tablets are 25mg, but check to box to make sure.