As a Mother to two autistic children, having Autism myself, YANBU!
My son is 12, my daughter is almost 10 and both would have no problems walking this distance. Does your ds enjoy school? Could he be displaying extreme anxiety about going to school when he is with your ex? My ds would not be able to leave the house if he had to go to school, but we home educate him. Could there be a bullying problem?
Unless your ds is frequently being picked on by bullies, then your ds should be able to walk that distance. Unfortunately, kids can be cruel and I often wonder if we autists have targets on our backs, that only neurotypical assholes can see. My ds has actually managed to develop coping skills when out with our dogs, but our dogs are very good at telling when someone is up to no good and have chased off bullies more than once (not literally but by a low growl and bark). Our ds takes our dogs everywhere with him but even so, we live in a very quiet coastal village in Scotland (so apart from local bullies there is little danger here), ds usually has a phone with him just in case.
Is the area your ds walks through safe? Or are there gangs where drug deals and knives are the norm? I would be concerned if it were the latter, particularly as bullying vulnerable children into running drugs is a problem in some areas (I am pretty sure that was in a recent NAS publication). I assume however, that you would pick your ds up yourself if he were in any danger!
Could your ex be trying to make you look like you don’t care about the well-being of your ds- could he be angling to get full custody?
In your situation, I would sit your ds down and ask him if anything has happened on the walk home (at any point, not just this week or today) - from or to school- eg bullying. I would also ask your ds if the walk to and from school is causing him anxiety. It has taken several years to teach my ds that he needs to tell me if something happens when I am not with him, as he would assume that as he knows what happened, that I automatically know it too.
The last thing you want is to end up in court hearing your ds tell a judge that the walk to and from school fills him with anxiety due to his experiencing frequent bullying and manipulation from bullies/ gangs.