If you notice he loses a lot of weight or his appetite is poor you can get Hills AD invalid food from your vet, it's very tasty and soft, and can even be made soupy so it can be spooned in.I think there's something called liquivite that is a soupy , highly nutritious stuff too. You can get great supplements such as nutridrops to go along with a sensible high quality diet to try and maintain him.
I'm so sorry, the news must have been such a shock.

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