Your autistic child being so skinny is often a result of restrictive eating habits or even food selectivity.
Some autistic kids have restrictive eating habits and food selectivity issues, which can limit the child's nutritional intake.
Other reasons for an autistic child being so skinny are side effects of medications, high activity levels or even gastrointestinal issues.
If you're worried you should speak to the child's pediatrician to rule out any medical conditions that could be resulting in the autistic child being so skinny and not gaining weight.
Some things you can do are to increase your autistic child's calorie intake and create a positive meal environment, offer foods with small and incremental changes.
Many children that are autistic have a narrow range of foods that they accept to eat and are more picky than some children.
Some autistic children only eat foods that they like based on the appearance and acceptance, which can also lead to nutritional deficiencies.
Some medications that are used to manage a child's behavior can cause weight loss and or a lack of appetite.
And some children including autistic children develop eating related behaviors, like eating too slowly or engaging in excessive physical activity, which can affect the child's weight.
And in some cases, some kids including autistic kids and non autistic kids are naturally skinny and may not gain weight, no matter what they eat.
I have a 7 year old son that is high functioning autistic and he eats a lot but he's skinny and doesn't gain any weight other than when he grows.
His doctor said he was normal and as long as his bones are not showing that it's usually okay.
But it would be a good idea to take your child to the pediatrician to be checked out, just to be safe.