Awh he’s a big lop, mines tiny. I don’t have any seniors yet but maybe this can help.
As for his health, have you taken him to a rabbit savvy vet to make sure his teeth are good(back teeth)? Also maybe some X-rays or a CT can help see if there is anything serious inside of him causing him his troubles. Is he still pooping good and peeing good? I know a lot of UTIs can cause poor litter habits that lead to soiled butts and not using the box.
I hope this helps

My REW bunny is 13lb and she is at least 7-8 yrs old. I know it can be worrisome when you read about age ranges for larger breeds.

Did he have x-rays to confirm arthritis or could the vet feel some changes? Just thinking an xray might also be able rule out things like bladder sludge or stones.
Have you seen any changes since starting the metacam? Maybe another NSAID would suit? Do you think he is aware when he need to toilets at all?
Another possibility is E.Cuniculi, which can cause urinary incontinence. Rabbits can go most there life carrying this with no symptoms. But t present when a rabbit becomes older or has another condition challenging their immune system.

Have you noticed any change in how he moves about when he is mobile? What about when outdoors? Is he less confident?
I noticed a very sudden change in my rabbit when he reached age 5 or 6 due to some loss of vision. He was suddenly not confident in jumping down from a place he’d had no trouble with previously. At first I thought it was due to something musco-skeletal, but generally, his activity levels had not declined. Turned out it was his eyesight! Once I put a ramp in place, he was up and down the level without any issue.