I don't have experience with stomatisis however we do (my parents) have a fur baby , Fibrizo, with a permanent ulceration in his gums that has caused him a lot of pain through his life. We give him coconut oil and it seems to help him but he didn't present any of the other symptoms that you mention with Simon. More of their cats are on coconut oil and it has helped them from bad breath to joint pain. As I said it helps it's not a magical panacea either but it sure has a lot of useful properties.
I would talk to your vet because the lack of voice could be a respiratory infection that maybe was oportunistic due to Simon's problem. Just a theory. Could be the oil. Yoshimi had a voice problem and it turned out to be a viral disease but she was treated, now my white fluff is fine. So as you see there could be many reasons and poir Simon is in pain.
Get better Simon, stronger and healthier