What Causes Sore Throat After Angioplasty?

My father after having a heart attack and angioplasty went home and started to develop sore throat which eventually got tso bad I had to take him back to hospital. He had developed sores on one side of mouth near the back to the point he could not even swallow water and had lost a good 10 lbs over the course of a month. now he had been in hospital 8 days on Iv steroids which elevate his blood sugar to 487 and then needs insulin. whenever I ask what he has no one will give me a straight answer , they have tried antibiotics to no avail then steroids which do give him temp relief. How can I find out what he has?. Linda