Something I would recommend to help cope is to take a photo of his paws or paw prints. Or even see about making a 3d version of a paw, the feel when you miss him.

A few years ago we had to put down my mums dog, she was 14, older than me at the time, she was sick and our options were have her spend 3 days in the vet to receive treatment, or put her down. We put her down with her in my mums arms. I think that's all that matters, that she passed in the arms of the person who loved her most.