They are very good at telling you when it is their time. I would suggest however, if it is a bone issue, maybe to not give him time outside, it is pretty cold out now and cold and damp makes bone issues quite painful. I have a boar currently who has Osteodystrophy and even with the oil filled radiator and his little snugglesafe heat pad, he can still have bad days where he's sore if he gets chilly overnight. I would see how he goes once kept warm with no time outdoors, just to see if that makes a difference to his overall demeanour. The joint pain will have made him reluctant to move about after going to the toilet and they quickly manage to get themselves in a mess, on Leonards bad days he needs a good wipe down with a warm damp cloth and dried. I have some F10 barrier cream too which stops urine scalding him and bacteria irritating his skin
Wishing you both the best, whatever decision you make, it will be the right one. These decisions when made with your pets best interest at heart are never wrong.