Photoshop - website banner image

Hello, I have an image of a dog (my dog) that i need cutting out of an image. I've tried very hard to do it myself but can't get it to work properly. When the picture of the dog was taken, Theo was laying on a duvet. Essentially, I want to place him on a blank background (light grey) and use it as part of the banner on my homepage. I have attached screenshots to show what I mean. Can anyone help?
I need this to be turned around rather quickly and am hoping it's not too much work for someone who has better photoshop skills than me!