If it is not possible to wash him, then tayammum (“dry ablution”) may be done for him, because of the general meaning of the aayah (interpretation of the meaning):
“So keep your duty to Allaah and fear Him as much as you can”
and because Allaah has prescribed tayammum for purification from both major and minor impurity in cases where water is not available, or where it is not possible to use it, or where using it will cause harm.
And Allaah is the Source of strength. May Allaah bless our Prophet Muhammad and his family and companions, and grant them peace.