I wouldn't worry about the "fun shrooms," worry about the ones that can and DO kill. Have you taught a very strong "leave it" command? The way we do it in class is to have a yummy thing where the dog can get it, walk up to it, then when the dog goes for it, give a good "LEAVE IT," then quickly move away, praise profusely when the dog moves away and give some really good treats to enforce that leaving whatever it was alone yields much better goodies. If I see my dog go for something potentially bad, my voice is full of the urgency that says "LEAVE IT ALONE- IT CAN HURT YOU" and he does immediately spring back, more from the tone of voice.
I wonder if you could set up a scene outside with some good. safe mushrooms from the store, then teach "leave it" that way? I haven't done it, but I wonder if a safe set-up like this would go a long way to teaching him that anything that looks like a mushroom is a big no-no?