This can be done, but you would have to be very careful.
Not knowing exactly what it is about his conscience that you want to affect, I can only give a general answer.
Say for example you wanted a person to notice the bad effects they were having on others by their actions, you could cast a spell to open their eyes to those effects.
This sort of spell would cause the person to see what their actions are doing to others. It would still be up to them to correct their actions, but if they were unaware of the effects their actions are having, it could be enough to cause them to change their actions.
