What is happening at the disabled day centre you take him to?
Maybe ask them if all the users and all the carers and all the parents, and the safety inspectors, and insurers, have agreed there is absolutely no risk of infection ever again, no matter what is done by the other users and their families, and all the people all those families mix with?
This is appalling, that you and anyone you mix with is being put at risk of potentially fatal illness by orders of an employer.
Equally bad is that other users of that centre a) will be at needless risk, and some, no doubt, will be extremely vulnerable b) the centre has obviously not impressed on every user, and everyone in contact with every user, the fact that there still is such a thing as Covid 19,. It has not gone away, and there is no such thing as a person who is not at risk of getting it. Nor anyone who is not a potential infected spreader of virus, because many people have it even though they have no symptoms.
You are doing an important job. But you are not a soldier, and did not agree to risk your life. The other posters are right, about keeping windows open during the drive and of course mask wearing. The woman is lucky her son is taken on such a long journey and entertained all day, despite having, as you say, only mild impairment.