Has your dad or girlfriend tried to express to the child that if they keep it up the dog might bite them? Does the dog actively seem bothered when this kid is doing this stuff to him, or does he more just try and ignore it? I find it a wee bit strange an autistic person wouldn't be more in tune with the dogs body language, being on the spectrum myself I think its much easier to understand animals than humans.

But yeah if the dog is very bothered by this and tries to fight him off, it is possible he might bite out of feeling threatened. If the dog seems 'irritated' but tolerates it without much struggle he probably won't bite unless the kid did something really out of the ordinary and scary like if he suddenly started hitting or kicking the dog. But I have seen dogs annoyed but not threatened by some kid all over them grabbing and pulling seems they just hope it ends soon so they can go back to being a dog, and I have seen dogs that get freaked out by that.

How does this child respond when told not to pet the dog that way?