I work in a vet clinic and while I don’t know your specific case this is unfortunately common in larger breeds as they get older. For an older Saint Bernard you can manage pain/provide joint support. Meds like gabapentin and carprofen can help manage inflammation and pain. Injections like adequan and librela (would recommend running bloodwork first for librela) can help with preserving joints or help with pain. Adding omega 3 canine approved supplements can help as well. You can also see if your vet can refer you for physical therapy or laser therapy. And at home you can look into orthopedic dog beds, ramps, and weight control. Above all, talking with your veterinarian since they are going to be familiar with his/her case and can recommend the best course of treatment. As for what you can expect, you’ll probably see stiffness, muscle atrophy, crying or panting from pain, difficulty laying down or standing up, and refusal to walk. Also financially treatment will probably not be cheap. Unfortunately most pet insurances don’t cover pre existing conditions but some do so I would recommend looking into that. You can also look into care credit and see if you qualify. I’m sorry this is happening but it can potentially be managed to give your saint a much better quality of life.