This may be true for people who are simply picky, but rarely does someone go for so long "just" as someone who is picky. Autism is a neurodevelopmental disorder and influences behavior in ways which override many of the body's instincts such as what you're describing. It interferes with people's interoception, which is the brain-body signalling process that tells you you're thirsty, hungry, you should move around, etc.

ARFID (the typical autistic eating issue which I suspect might be going on with OP's kid) is not simply pickiness, but a distressing aversion which can make it nigh impossible to swallow "unsafe" foods if you're not actively working through the disorder with a therapist. People with these kinds of sensory issues experience panic attacks and terror induced by textures, scents, etc. Depending on severity it can be a choking hazard. It is certainly something that can be worked on but it needs to be done very carefully.