My kid has ARFID -
1) Understand that there is a sensory component to this and don't brand the kid "picky" or difficult. They can't help it. Also, you are not a "failure" as their parent. Take the emotion out of it. My kid is older but their therapist said that me asking my kid to try rice as though I had put a bowl of vomit in front of her. Don't make meal times fraught and stressful for the kid (and yourself).
2) Getting them to eat - whatever it may be - is the most important thing. So give them what they WILL eat, without judgement.
3) Over time, your child can work with a therapist or OT to broaden what they eating through "chaining" i.e. if you like pasta with butter, try a different shape, then eventually try red sauce, etc.
4) Chocolate milk and/or boost could be a way to get calories into your kid.
Good luck - this is hard stuff! What makes it even harder is that people judge you and your child - they assume the kid is manipulating you, and you are a pushover.