Online questionaires are not a good idea for diagnostic purposes. Only a psychiatrist could diagnose a complex criteria like anhedonia or depression itself. If he has a loss of interest in life it could be due to many things. A support group would be a good idea and he should be encouraged to see a talk therapist. They could decide if a further referral to a psychiatrist is needed. He may very well have depression but a psychiatrist has to understand specifically what is going on in his life in general to able to provide a diagnosis.
This does put a strain on a relationship, and with him refusing help, must be very frustrating for you. It is best treated with antidepressants and psychological counseling. Since he will read, I would get him books on the topic and once he educates himself on this, he may see the need to get a professional opinion and help. He shouldn't assume that this is what he has. People with severe depression experience no pleasure out of life, and I feel there is a fine line between the two. Don't fight him on this, get him the books, and I truly feel the more he learns the more open he will be to help. He may just be in denial right now, and once he learns more will feel better about seeking proper help. Good luck to both of you.
This anhedonia depression you refer to is not an illness in itself at all.
Rather it is describing just one key symptom of depression, the inability to enjoy, to find joy. It is usually accompanied by a range of other symptoms such as tiredness, loss of motivation, concentration and much more. Sadness of course. A want to isolate himself too.
It is also a symptom of schizophrenia so, to me, he desperately needs to see a doctor. You know this.
If he has decided this on his own it is likely he has missed quite a few things that would allow a professional to give and accurate diagnosis.
A lot of men are afraid to admit they have depression simply because society puts that pressure on us, unfairly. I say if you have it scream and run to the doctor. Else you are punishing yourself and your family for no good reason. I hid it for a quarter of a century so I'm no role model.
Tell him that and tell him I think he must see the doctor else he is a fool. Let him read this so he can get angry at me. That may spark him to prove me wrong and go see a doctor to do that. I dare him! He won't will he, he's too scared of the truth.