I would recomend seeing the vet. There are many reasons an "older" dog might slow down, arthritis is only one of them. If he does have arthritis, there are several medications that help quite a bit, but your should have a vet guiding you, as there are side effects you need to monitor. The other thing that is common in older large dogs is back pain, which may be caused by disc disease. My golden retriever has both arthritis and disc disease, and he has had great success with arthritis medicine (meloxicam) and accupuncture for his back. Your vet will examine the dog, and might want to do some basic blood work, and X-rays depending on what they find on exam.

Good luck.

Jake's mom