Cat Excessively Biting Penis (Not UTI/Crystals)

Hello! I am posting this to see if anyone has ever encountered anything similar because I am at the end of my rope and my cat's life is actually in the balance.

I have a 6 month old male neutered cat that is excessively licking and chewing his penis. He growls as he does this and he has actually done it to a point where it bleeds and bruises and he cannot urinate properly. He has been to the vet and does not have any other health issues. He does not have an infection of any kind and he does not have urinary crystals (verified through urinalysis) and he has had radiographs which showed no abnormalities. He had been on clavamox just in case, as well as buprenorphine for pain, and fluoxetine for any psychological component to his behavior. He was even kept at the veterinary clinic for observation. When I brought him home he was doing well for about 2 days and then resumed the behavior. He now wears an e collar constantly, but still becomes agitated and attempts to bite himself. My veterinarian is out of ideas and has said that if this continues he may need to be euthanised.

So my question is, has anyone experienced this before? Is there anything else I can be doing for this cat? I provide stimulation at home in the form of toys and treats and playtime and he eats the Royal Canin Urinary SO food, as that is what my other cats are on. I want to save him but I don;t know what to do. I would be so grateful for any help at all or any information I can pursue. Thank you so much!