Gus needs to purée his soup while it's still in the pot. what is the best tool for him to use?

Probably an Immersion Blender, as putting hot soup into a blender would break it and cause soup to go everywhere ;)

An immersion blender can be placed into the pot and won't break, as well as not make a mess.

He can use a masher which would be efficient yet still give it a good texture. I personally use this electric blade that purees it for me I think its a kitchen aid brand but idk