"why this cop would say the boy could not have done it"?

Possibly because the cop is hungry for a promotion or the whole PD craves positive publicity as being tough on crime, and crimes against children usually get more public attention and response, especially with a touch of a "sensation", which is added by the boy's having autism. Perfect case for them to be made, even from scratch if needed.

I do not know that mother either, whether she did or did not do it...just trying to answer your question using my best guess.