The issue is not the mushroom!
The issue deep down is he felt like he is no longer in control in his life. He is getting older, don't have the means to bring in money, don't have the language skill any more. He cannot get around unless someone bring him somewhere. He felt like he is a burden of the family.
When he compare the good old days in China, he should be the boss, the head of the family. In his mind, he is providing his family (bring in money like he once did). Your rejection is like rejecting his afford to provide.
Understand his feeling, and don't hold it again him. I bet he doesn't understand it either. Get him more involve in family affair, ask for his input that could make him feel better.
Unfortunately, not much could be done for him. Just wish when we were older we don't get into the same situation.