Cancerous mass on old horse?

My 27 year old gelding has a sudden mass that appeared on his cheek. It solid not squishy, no tooth problems or infection. Vet Xrayed it and were pretty sure it’s cancer. He’s been declining terribly in the last week he’s still eating but he’s having sudden symptoms that I’m wondering are related, I’m worried he’s losing sight in his right eye and tonight my poor baby could barley walk on his right leg, I can’t find any obvious inflammation or swelling and I was wondering if that could be related to the mass? We know we’re going to have to put him down but we didn’t think it would be this soon but if the mass is causing to many problems I think it’s going to need to be done sooner. So is anyone familiar with this or dealt with this before?

My sweet old man. You can see the lump on his right cheek.

UPDATE

Well he’s still going strong and eats everyday, he still half runs around the pasture and rolls in the sunshine. I guess we will see from here. It’s so hard to make the call when to do it.