
We have had two occasions when we had singleton puppies to rear, the first being my first litter when due to my inexperience, we lost the first out. The second litter only ever was one. With the first, having read all the books, I thought I had to be piling in the food (mum) when actually nature knew best and she really didn't eat very much at all. Which was to be expected. Singleton, or even a small litter = mum doesn't need to be piling in food. Provided you rotate the puppy across all the teats, you should be able to avoid mastitis, but do keep checking because, again with too much food, she may be producing more milk than one puppy remotely needs. I'd get her back onto her normal maintenance diet.
As for him, all alone in the nest once mum decides not to spend much time in with him, which will probably happen, I stuffed a sock to lay alongside him for comfort. And I have to say because I felt so sad for the single puppies, I carried each of them, each time, around with me. The male, from the first 'litter', we sold and he went off by 8 weeks as I knew the people well, and he was doing fine. With the second singleton puppy, she was a bitch and we kept her. And she became really spoilt because of all my attention early days. BUT we had another litter that same year (unusual for us but that's how it happened) so once we were down to the one we were keeping from that litter, we had him in with the singleton bitch so she had company. We couldn't put our young Bassets in with the adult, for size difference alone.