Autism can't be medicated, all autistic children are different, what suits one for calming won't suit another.
However, if you want to try and lessen the child's anxiety, then that should - note should because nothing works 100% of the time and triggers aren't singular, they can mount up until the pressure is too much - lessen the more extreme autistic behaviours.
Basically you need to identify that child's triggers and when you can, put interventions into place so the triggers don't distress the child.
Easy example, noisy environment, use ear defenders.
More difficult example, family party, noise, people behaving differently, not understanding their place in this social activity - careful planning beforehand, explanations of what's going to happen and a quiet space just for the child before they become overwhelmed.
Things known to be calming are one or two drops of lavender essential oil in a bath or on a tissue in the bedroom, check dosage instructions and possible allergies before use.
Does s/he find any type of touch soothing and calming, if so build that into their day when they need it.
Small drink of weak chamomile tea if they like the taste, strength dependant on age. e.g for a junior school age child, one third of an adult strength tea.
Sensory Tent, you don't have to buy a purpose made one, you can make your own from a cheap pop-up tent, pretty battery lights, soft cushions and things with textures the child feels are relaxing, maybe a couple of larger soft toys where you add lentils or dried beans or rice to make the toy heavier to act like a weighted blanket, plenty tables online re what weight of child and blanket.
Some children like to rock or spin or be upside down, some like firm pressure, some like quiet, some like classical music, some like darkness, they are all different. It's all trial and error, but you'll create a calming atmosphere for your child, that's what matters.
Lots of sensory info here www.falkirk.gov.uk/services/social-care/disabilities/docs/young-people/Making%20Sense%20of%20Sensory%20Behaviour.pdf?