Fear of food poisoning (emetophobia) and best treated with CBT. A nutritionist might help, and speech & therapy work with swallowing difficulties.
I’ve worked with autistic pupils with restricted diets previously and it takes a long time. With one who would eat one thing for weeks then go off it and want nothing, I made a Velcro board and he planned his good choices for a week with his mum on a Sunday. He was allowed to have the favoured food 4 times a week and had to choose something else the other, so there was always another option of he went off the favoured food. This pupil would gag at the thought of a school dinner but I made him look at them every day, after about 2 years he would taste the pudding some days after 3 he would have a small portion of chips.