Though I agree with the overall drive of the comments here (the child needs to be taught that batteries aren’t safe to chew) the lack of empathy for the parent of an autistic child who is has been pushed so far as to ask for this help is astounding.
As a parent of an autistic child who fights similar battles I feel for this person. I can’t put words in peoples mouths, so here’s my own interpretation:
This person more than likely knows this isn’t the right answer but is at their wits end and looking for the least damaging option. Rather than berating them for something they likely know is bad please realize this is the tip of an iceberg and they are quite possibly in damage control mode. Some of the comments here (like the chewable necklace) are great. Others (the many who are berating the parent for even considering this) missed the larger issue at play.
OP: We deal with similar things and we’ve found wooden toys to be the best alternative. Our son used to love the silicone chew toys but now wants something better so we seek out US/EU made and well regarded (from a chemical perspective) wooden toys so that if he does chew it doesn’t hurt him. There are some things that are just a forever fight to keep away from him. Specifically for batteries he doesn’t touch them now because he associates them with being needed for his Leap Pad so that’s a higher priority for him, so creating a higher need like that is a possible way as well.