I am sorry to hear you are worried about Wilf.
My name is Rebecca I am a veterinarian, and will do my best to help.
If he only ate part of it, he will probably be OK. However, there are some really poisonous mushrooms, as you probably know.
I am not an expert at mushroom identification, and even experts sometimes get it wrong.
You could try to get him to vomit if you are worried.