I am sorry to hear you are worried about Bristol. I am a veterinarian and will do my best to help.
You can use topical triamcinolone, but very sparingly. It is a strong steroid, and can be absorbed systemically.
Tea Tree oil can be toxic
in dogs, so good that you stopped it.
Itchy feed often reflect a systemic problem, like allergies. You could try giving antihistamines like Benadryl or Claritin. How much does he weigh? I could tell you a safe dose if you tell me his weight.
There are supplements that help some dogs with itching: fish oil, evening primrose oil, probiotics, nettles, quercetin, anti oxidant vitamins.
Oatmeal bathing can also be helpful.
Let me know what else I can answer.