Is it permissible for a son to perform Hajj on behalf of his parents if they are still alive?

It is not permissible to perform Hajj on behalf of a living person unless they are incapacitated (unable to perform Hajj themselves), provided that the person performing Hajj on their behalf has already performed Hajj for themselves. So, if your parents are incapacitated, there is no harm in performing Hajj on their behalf, if you have already performed Hajj for yourself.