Hi there, I'm Dr. Sara. I'm sorry to hear that Willy is having difficulties with his hearing. I know that i can be scary and frustrating when you aren't entirely sure what is happening.
Unfortunately, without doing some more in depth diagnostics, it's unlikely that your vet will be able to tell what the cause of Willy's hearing loss is. If he had recently had any ear infection or discomfort treated with topical medications like ointments or flushes, that could rarely lead to hearing loss. If it's a medication related hearing loss, it usually happens during the course of treatment. Sometimes the hearing comes back, sometimes not - the only way to know is to wait and see.
If there was no recent ear infection or medications/flushes applied to the ear, then we are unlikely to be able to pinpoint the problem without sending you to a specialist. A test called a BAER can give some insight into the location of the lesion that is causing the loss of hearing, and if we really wanted to get into it an MRI might give some info as well, but most vets (and owners) would rightly consider an MRI overkill if the dog is apparently healthy otherwise but just deaf.
If he has any other neurologic symptoms like seizures, vestibular or walking difficulties in the next few weeks to months, then the hearing loss may have been the first sign of something more insidious like a brain infection or tumor - but this is quite rare, especially in a young dog.
I'm sorry to say that most of the time if the only sign is hearing loss and the dog is otherwise healthy, we often don't get an answer or diagnosis other than "idiopathic", which means "unknown cause". An exam by a veterinary neurologist may be your next step if you're wishing to pursue more in depth diagnostics.
I hope this helps - please let me know if I can answer any other questions for you!