I obviously don’t know what was said but as the op has come on here for parental advice not public castigation::
~ he may be legally an adult at 18 but his
pre-frontal cortex won’t be fully formed at this age which will make him prone to impulsivity
~ ASD (as it is now) will make him more likely to be easily lead by others and he won’t fully understand the impact of his words on others
So it’s reasonable that both his age and his ASD should be taken in to account when disciplinary measures are meted out.
Op, as a pp said, I would check out his role carefully in relation to others in the group
and if he hasn’t been in trouble before, and genuinely now sees the error of his ways, then he would do well to write a sincere detailed letter of apology to the girl involved, written out in long hand, giving reasons as to why he was wrong, and passed via the head teacher to the girl’s parents. And keep a copy for yourself.
You may need to seek the help of a lawyer if police become involved, so get a recommendation for someone who deals with educational matters in your area.
And ignore any hostile comments on here.
You are under no obligation to broadcast what your son said. At one time you could come on Mumsnet and acknowledge a fault in yourself or one of your dc, and not be flamed but those days are very sadly gone and responses are less measured.