I'm really sorry, a horrible day, but you recognised it was time, made the decision and did it. Not at all easy for you, but best for him and that's why we do it.
So sorry that your lovely boy has gone. It's even harder when they go earlier than you'd expect. He was lucky to be so loved, and that you made the kindest decision before it became really bad. Bone cancer is awful even as cancer goes, such a cruel disease. I'm glad that you've found a whippet puppy for your mum. They're such lovely dogs, and I'm sure he/she will make her grief a little easier to bear. Sending hugs to you both. xx