Here is a fact sheet:
https://www.alzheimers.org.uk/site/scripts/documents_info.php?documentID=154
You could choose to have joint and several powers with your son, or have him named as a replacement attorney, as nitram says.
In my experience it is best to have belt and braces, and not risk a situation where someone has to apply for deputyship. My mum thought she had covered all eventualities but I still ended up having to apply to the Court of Protection because her arrangements failed. When I sought Guardianship of my mother I asked my OH to be joint Guardian, and our eldest son to be replacement Guardian, to cater for my incapacity or demise, or indeed that of both OH and myself.
For my own LPA I have named OH and all 3 of our children with joint and several powers. In practice they probably wouldn't all want to get involved, but at least I haven't shut any of them out as my mother did with me, only to arrive at the situation where I was the only person willing and able to look after her affairs.