Stevens-Johnson syndrome caused by immunotherapy trial

My husband was recently diagnosed with Stevens Johnson Syndrome and
Toxic epidermal necropsy is overlap syndrome due to immunotherapy treatment for his esophageal stage 4 cancer
The rash began after second infusion and was treated with prednisone - a low dose version not to interfere with treatment. The rash began to spread over his entire body. Several doctors saw him while this was worsening and in spite of our constant questions and concerns failed to address the problem until it was beyond control when they then took him off the immonotherapy regimen - sent him to Pallative Care who increased his steroids with a virtual meeting and then referred him to hospice with no diagnosis
It was only thru an emergency room visit that he was diagnosed
How does this happen? We feel totally lost and confused
Hospice was kind and I believe will help him to heal but we are without help now for his cancer until he does heal and we feel oncology has pretty much written him off here
Pallative care response was they had never seen a rash as severe and any help they would give us would be only a trial and error - but they made no mention of this syndrome
Like I said we are very confused and feel like no one really is supportive and we are drowning - he is not ready to give up
If anyone could please help