Sorry, I missed this topic.
I had one who was on a diet (hill's S/D) for 6 years. He was blocked twice and nearly died both times. It appeared his penis was too small. I too knew someone who had a cat whose penis was amputated for this problem, it cost them an awful lot of money.
As a result of the pain, my cat didn't use the litterpan and peed in my hall. I cleaned twice a day so the neighbours must have thought I was obsessed
In the end I could not take it any longer and took him to be PTS. He was 7 and the vet said I was doing the very best as he was really ill, but I cried my eyes out. I felt so bad. It didn't help that people said I was an angel for coping all those years.
So good luck for Arty and you