The moral calculus here depends on how severe the impairment is and the potential risks / harms associated with the treatments.

At one end of the impairment/risk ratio, it is certainly ethical. Somewhere in the middle, I think you need to obtain the minor's consent. At the other end the treament isn't ever ethical ever to conduct on a minor.

What if the "sight saving" treatment has a high risk of them losing their sight at an older age when developing the abilities to navigate as a blind person are harder?

The issue here is an ongoing debate about which ASD differences are impairments (and about the severity.) and which are just stigmatized. I think to make this determination ethically you MUST listen to the range of opinions from adults who faced similar situations. If there isn't a clear consensus, I think we should be hesitant to impose treatment without consent.