Itchy skin relief from allergen?

Over the holiday I boarded my dog with someone through Rover. I’ve done it many times before, but not with this person (they were well rated). Things seemed fine, updates said he was being good. When I went to pick him up last night (30th) they said that he’d had issues pooping and had called the vet and I guess was told to do the chicken and rice method, which they did. This was never mentioned to me until pickup.

The issue is that my dog has a chicken allergy, which I’d told them about and confirmed at that moment that they knew. They said that it was only once and that it was a small amount.

Well I get home and am loving on him and checking all over and I notice scabs, marks from chewing on himself, redness between his legs and overall being very itchy (petting his back gives the distinct itch cringe). I gave him a little Benadryl to help him sleep, but it wasn’t enough because I woke up around 3am to him scratching, to the point that his chin/neck was bleeding a good amount.

Asked the sitter and they said they gave him the chicken on the 26th, but I don’t know what else would cause such a reaction (apparently though it can take a while for the allergen to clear the system). They also gave him a flea and tick bath without my knowledge, despite him not having any and being on Simparica Trio.

Sorry for the whole backstory but I needed the rant because I’m furious that he’s been made so miserable. What can I do to alleviate his suffering until it clears? I figure an oatmeal bath but I also don’t want to bathe him too much if it dries out his skin. Thanks for any advice!

TLDR; Rover sitter gave my dog chicken, which he’s allergic to, and now he’s itchy and miserable and needs relief.

Other notes: -He’s a ~75lb black dog mix (so skin issues are genetic. -I know he’s allergic to chicken because he used to get itchy (never this bad) until I made sure he had chicken-free food. -I’ve told the sitter the issue and they’ve offered to refund the stay, but I’m still debating on accepting that.