Would it be okay to take down my son's door?
Why do you want to take down your son's door?
he thinks it's a few blocks too high.
It's great that you want to help your son adjust his door. Helping your child is a good thing and will help to increase the bond between both of you.
So you approve of me taking down his door?
Sure, help him adjust it to the height that you both find appropriate. Measure it carefully and put it back up as you see fit.